Migrate crashlytics tests to JUnit4

[daymxn]

Per b/375055031,

This performs various cleanups on the integration tests for crashlytics. This should help narrow down flakey test behavior, and avoid any issues that may arise from deprecated usages.

Namely, this PR does the following:

• Fully migrates all the tests to JUnit4. Some of them were still using JUnit3 behaviors.
• Migrates initMocks to openMocks. There may have been broken behavior from initMocks leakage.
• Updates some existing openMocks to properly close. There may have been leakage into other tests, which could cause false positives.
• Migrate AndroidJunit4 usages to the new import. Some of the tests were still using the old (deprecated) version.
• Suppresses the receiver filter warning. This is a false positive as the use-site only specifies two arguments. By suppressing the warning- it should help depollute our logs.

[github-actions]

Release note changes

No release note changes were detected. If you made changes that should be
present in the next release, ensure you've added an entry in the appropriate
CHANGELOG.md file(s).

[github-actions]

Vertex AI Mock Responses Check ⚠️

A newer major version of the mock responses for Vertex AI unit tests is available. update_responses.sh should be updated to clone the latest version of the responses: v5.1

[github-actions]

Test Results

   72 files  + 8     72 suites  +8   6m 40s ⏱️ +28s
  509 tests +22    509 ✅ +22  0 💤 ±0  0 ❌ ±0 
1 024 runs  +44  1 024 ✅ +44  0 💤 ±0  0 ❌ ±0 

Results for commit 12ef52c. ± Comparison against base commit d402ad0.

♻️ This comment has been updated with latest results.