feat(@angular/cli): option to build and run only specified spec files

[FDIM]

A new --spec option is added. It is a glob/pattern that specifies which specs should be included in the build.
e.g. ng test --spec=app/services/auth* (.spec.ts is added automatically if missing)

When karma is already running, selected spec can be updated by running ng test --spec=app/login* --spec-update

With such option it is also trivial to add custom vscode task to be able to run currently open spec file.

Behind the scenes a new test.generated.ts (could be temp?) is saved next to original test.ts which instead of using require.context imports all matching files. This gives quite a big boost in the project at work (at least 10s on each iterative build!)

Fixes #3603

Motivation: It is a pain in a big application to wait until entire app is rebuilt just to run single fit block. Takes almost a minute with 4000+ tests and barely works when code coverage is enabled.

Missing: docs(?) and tests

Disclaimer: this is quick and dirty solution that can be improved a lot but some feedback before making it nice would be great!

[ngbot]

Hi @FDIM! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

[mgechev]

@FDIM would you add e2e tests for the feature?

[mgechev]

Closing it due to inactivity. Please reopen if you come back to it.

[FDIM]

@mgechev I've updated the branch - refactored it a bit, added tests and I think it's ready for review

Edit: looks like I can't reopen the PR - will create new one

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