Add e2e tests for namespaced install of ClusterExtensions

[camilamacedo86]

The support to allow namespaced install was added in: #1724
We need to add an e2e test to cover it.

[camilamacedo86]

@perdasilva Have we it done as well?

[camilamacedo86]

Closing it as done we have e2e tests for it.