Fixing some Firestore test warnings, caused by failure to find ADC

[hiranya911]

This is to fix some warnings observed in the Travis CI environment (which does not have application default credentials set up).

Before the fix:

Firestore
    Initializer
      ✓ should throw given invalid app: null
      ✓ should throw given invalid app: null
      ✓ should throw given invalid app: 0
      ✓ should throw given invalid app: 1
      ✓ should throw given invalid app: true
      ✓ should throw given invalid app: false
      ✓ should throw given invalid app: ""
      ✓ should throw given invalid app: "a"
      ✓ should throw given invalid app: []
      ✓ should throw given invalid app: [1,"a"]
      ✓ should throw given invalid app: {}
      ✓ should throw given invalid app: {"a":1}
      ✓ should throw given invalid app: undefined
      ✓ should throw given no app
      ✓ should throw given an invalid credential with project ID
      ✓ should throw given an invalid credential without project ID
      ✓ should not throw given a valid app
      ✓ should not throw given application default credentials without project ID
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 143): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 144): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 145): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 146): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 147): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 148): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 149): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 150): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 151): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 152): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 153): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 154): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.
(node:3248) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 155): Error: Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.

After the fix:

 Firestore
    Initializer
      ✓ should throw given invalid app: null
      ✓ should throw given invalid app: null
      ✓ should throw given invalid app: 0
      ✓ should throw given invalid app: 1
      ✓ should throw given invalid app: true
      ✓ should throw given invalid app: false
      ✓ should throw given invalid app: ""
      ✓ should throw given invalid app: "a"
      ✓ should throw given invalid app: []
      ✓ should throw given invalid app: [1,"a"]
      ✓ should throw given invalid app: {}
      ✓ should throw given invalid app: {"a":1}
      ✓ should throw given invalid app: undefined
      ✓ should throw given no app
      ✓ should throw given an invalid credential with project ID
      ✓ should throw given an invalid credential without project ID
      ✓ should not throw given a valid app
      ✓ should not throw given application default credentials without project ID
    app
      ✓ returns the app from the constructor
      ✓ is read-only
    client
      ✓ returns the client from the constructor
      ✓ is read-only
    client.projectId
      ✓ should return a string when project ID is present in credential
      ✓ should return a string when project ID is present in app options
      ✓ should return a string when project ID is present in environment