Add emulator-based integration tests.

[yuchenshi]

Hey there! So you want to contribute to a Firebase SDK?
Before you file this pull request, please read these guidelines:

Discussion

This is a follow up to #1148 and enables emulator-based integration tests.

Testing

This covers Firestore and most of Auth and database.

API Changes

N/A

[hiranya911]

Thanks @yuchenshi. Really like how this is turning out. My main point of feedback is about managing the emulator-based integration tests entirely within the Actions workflow, rather than in package.json. That's consistent with how we did this in Python, and keeps the package.json file clean of changes.

[yuchenshi]

Sorry I was OOO and I've now reverted package.json changes as requested.

[hiranya911]

Thanks @yuchenshi. LGTM with a couple of nits. Feel free to merge once the comments are addressed.

[yuchenshi]

I see one flaky test here and I'll investigate later. Don't merge for now please! https://github.com/firebase/firebase-admin-node/pull/1155/checks?check_run_id=1881208101#step:7:197

It's unclear if it's flaky due to some issue in Auth Emulator or maybe the test itself has race conditions that got worse since Auth Emulator is much faster than prod.

[yuchenshi]

Merging now since the tests now pass. Feel free to skip that one test for emulators if this comes up again. Let's at least have coverage for the other tests.