Add tests for _initAuthResolver in FirebaseAuth

[PezCoder]

Description

src/auth/FirebaseAuth.js contains a function named _initAuthResolver
This pr adds unit test cases for _initAuthResolver function.
Reference: #951

[coveralls]

Coverage remained the same at 93.852% when pulling b346d24 on PezCoder:tests/_initAuthResolver into b6dca6e on firebase:master.

[PezCoder]

Not sure why the travis is breaking,
My observations for the second test:

1. ctx._initAuthResolver() will invoke auth.onAuthStateChanged internally with a callback to resolve the promise returned by _initAuthResolver function
2. Also our spy should be triggered & fakePromiseResolve should now contain a function calling which should call the above callback
3. Calling fakePromiseResolve & Promise should be resolved & expect should be checked, done should let the jasmine know that the task is done.

It's running fine locally but not on travis, maybe it's not resolving on travis.
Am I missing something ? 🤔

[coveralls]

Coverage remained the same at 93.852% when pulling 7ab47ed on PezCoder:tests/_initAuthResolver into b6dca6e on firebase:master.

[coveralls]

Coverage remained the same at 93.852% when pulling de454f7 on PezCoder:tests/_initAuthResolver into b6dca6e on firebase:master.

[PezCoder]

The newly added tests are passing as expected now, after i added tick() method at last. 😄
Buy how ? Also other tests that are not related to my pr are breaking.