Align Auth SwiftAPI.swift testing

[paulb777]

• Do SwiftAPI via standard unit testing instead of when building the integration test sample.
• Run API build test on all platforms instead of only iOS
• Matches location of auth-swift branch to make merging and alignment easier
• Continue to only test with CocoaPods until the library implementation moves to Swift
• Remove no longer necessary concurrency ifdefs
• See also auth-swift PRs [auth-swift] Remove erroneously added public api #12094 and [auth-swift] Mostly unified Swift API tests #12095

The changes to SwiftAPI.swift can be seen on the individual commit diffs

[google-oss-bot]

	1 Warning
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[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseAuth-iOS-FirebaseAuth.framework

  Overall coverage changed from 68.52% (5de0369) to 68.53% (caaf866) by +0.01%.

  Filename	Base (5de0369)	Merge (caaf866)	Diff
  FIRAuthKeychainServices.m	58.80%	59.66%	+0.86%

Test Logs

• Base (5de0369): https://github.com/firebase/firebase-ios-sdk/actions/runs/6829139193
• Merge (caaf866): https://github.com/firebase/firebase-ios-sdk/actions/runs/6830950546

1. https://storage.googleapis.com/firebase-sdk-metric-reports/iSt54CUq8L.html