[Firestore] Fix build-for-testing linking affecting CocoaPods (Approach 1)

[ncooke3]

Promising fix for #11973.

Global green test run: https://github.com/firebase/firebase-ios-sdk/pull/11977/checks?sha=35d90ffde73fc0e977e5881a4fd126df3d593e59

#no-changelog

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestoreInternal.framework

  Overall coverage changed from 88.15% (4b26b31) to 88.09% (28d765f) by -0.06%.

  Filename	Base (4b26b31)	Merge (28d765f)	Diff
  exception.cc	84.21%	23.68%	-60.53%
  leveldb_key.cc	98.14%	98.82%	+0.69%

Test Logs

• Base (4b26b31): https://github.com/firebase/firebase-ios-sdk/actions/runs/6591618439
• Merge (28d765f): https://github.com/firebase/firebase-ios-sdk/actions/runs/6593613353

1. https://storage.googleapis.com/firebase-sdk-metric-reports/N8d4tUDG5i.html

[paulb777]

Great find! Any reason for weak_framework vs framework?

[paulb777]

LGTM, when Gemfile is reverted

[paulb777]

We may need something like this more generally for #11842 (comment), but we can sort that out, post 10.17

[ncooke3]

Great find! Any reason for weak_framework vs framework?

Thanks! So that was informed from the docs for framework. The docs mention it's for system frameworks. It might still work, but weak_framework did not have that clarification.

Some other approaches I tried:

1. Programmatically linking the class. See the diff: master...nc/firestore-fix-2 This seemed to work, but we'd need to do it for every public Firestore symbol being re-exported, which seemed less than ideal. I also couldn't fully reason through why some of the bits in that diff were necessary based on my testing. This approach seemed to work though, I just thought this PR's approach is simpler.
2. Adding a s.pod_target_xcconfig with the s.pod_target_xcconfig = { 'OTHER_LDFLAGS' => -framework "FirebaseFirestoreInternal" }. This approach did not work in my testing.
3. Removing the -fno-autolink flag in the FirebaseFirestoreTestingSupport.podspec. This also did not work.

[ncooke3]

Thanks for the merge, will re-tag and stage.