[Auth] Add support for using the emulator for integration tests

[sam-gc]

This change does a few different things:

• Lets you run integration tests against the emulator (main change)
  • Using two new commands: test:node:integration:local, test:browser:integration:local
• Moves the node integration test spawning into a dedicated script (this is what Firestore does: https://github.com/firebase/firebase-js-sdk/blob/master/packages/firestore/scripts/run-tests.ts)

[changeset-bot]

⚠️ No Changeset found

Latest commit: 653b9d0

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[google-oss-bot]

Binary Size Report

Affected SDKs

No changes between base commit (57c73a0) and head commit (7110c0e).

Test Logs

• Base (57c73a02): https://github.com/firebase/firebase-js-sdk/actions/runs/611531534
• Head (7110c0e1): https://github.com/firebase/firebase-js-sdk/actions/runs/611900311

[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (57c73a0) and head commit (eb41ec6).

Test Logs

• Base (57c73a0): https://github.com/firebase/firebase-js-sdk/actions/runs/611531534
• Head (eb41ec6): https://github.com/firebase/firebase-js-sdk/actions/runs/611903079