refactor: Improve test directory structure

[demolaf]

Reorganizes the test/ directory structures for dart_firebase_admin and google_cloud_firestore packages.

[gemini-code-assist]

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[github-actions]

Coverage Report

✅ Coverage 73.79% meets 40% threshold

Total Coverage: 73.79%
Lines Covered: 4946/6703

Package Breakdown

Package	Coverage
dart_firebase_admin	70.92%
google_cloud_firestore	77.01%

Minimum threshold: 40%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.78%. Comparing base (4a4fc49) to head (d4d6552).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #192       +/-   ##
===========================================
+ Coverage   56.49%   73.78%   +17.29%     
===========================================
  Files          52      105       +53     
  Lines        3995     6703     +2708     
===========================================
+ Hits         2257     4946     +2689     
- Misses       1738     1757       +19     

Flag	Coverage Δ
unittests	73.78% <ø> (+17.29%)	⬆️

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