Firestore: Add @CanIgnoreReturnValue annotations in tests

[dconeybe]

Googlers see cl/457735855 from which this PR was ported.

[github-actions]

Unit Test Results

     78 files   -    314       78 suites   - 314   56s ⏱️ - 14m 58s
1 090 tests  - 3 609  1 074 ✔️  - 3 601  16 💤  - 6  0 ❌  - 2 
1 090 runs   - 3 625  1 074 ✔️  - 3 617  16 💤  - 6  0 ❌  - 2 

Results for commit c37de10. ± Comparison against base commit 921da2e.

[google-oss-bot]

Coverage Report ¹

Affected Products

No changes between base commit (921da2e) and merge commit (a7ba569).

Test Logs

• Base (921da2e): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-check-coverage/1544379313283928066
• Merge (a7ba569): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3872/check-coverage-changed/1544708610456555520

Notes

• Commit (a7ba569) is created by Prow via merging PR base commit (921da2e) and head commit (c37de10).
• Run gradle <product>:checkCoverage to produce HTML coverage reports locally. After gradle commands finished, report files can be found under <product-build-dir>/reports/jacoco/.

1. https://storage.googleapis.com/firebase-sdk-metric-reports/UI9edKEWO8.html

[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (921da2e) and merge commit (a7ba569).

Test Logs

• Base (921da2e): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-binary-size/1544379313283928067
• Merge (a7ba569): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3872/binary-size/1544708610460749824

Notes

• Commit (a7ba569) is created by Prow via merging PR base commit (921da2e) and head commit (c37de10).

1. https://storage.googleapis.com/firebase-sdk-metric-reports/AbrMZQba90.html