[auth-swift] Revoke Token unit tests

[paulb777]

Add Swift implementations for the two Revoke Token unit tests. This completes the implementation transformation of FIRAuthTests.m to AuthTests.swift

Two Revoke Token implementation questions came up while doing the tests.

• The old Revoke Token proceeds and the old tests depend on continuing with a nil idToken. I matched this in the Swift implementation
• The Objective C implementation was using the public getIDToken instead of internalGetToken. This added the requirement to use the main thread internally and made the test implementation difficult. I changed the implementation to use internalGetToken and only do the revokeToken callback on the main thread.

[google-oss-bot]

	1 Warning
⚠️	Did you forget to add a changelog entry? (Add #no-changelog to the PR description to silence this warning.)

Generated by 🚫 Danger

[renkelvin]

LGTM with 1 comment.