Fix: Improve Sentry Integration tests

[ueman]

📜 Description

This PR improves the tests which make sure that every platform uses the correct integrations.

#skip-changelog

💡 Motivation and Context

#390

💚 How did you test it?

I've written new tests.

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[codecov-commenter]

Codecov Report

Merging #421 (df2a62a) into main (bb04c3d) will decrease coverage by 0.39%.
The diff coverage is n/a.

❗ Current head df2a62a differs from pull request most recent head f66572f. Consider uploading reports for the commit f66572f to get more accurate results

@@            Coverage Diff             @@
##             main     #421      +/-   ##
==========================================
- Coverage   90.02%   89.63%   -0.40%     
==========================================
  Files          54       49       -5     
  Lines        1684     1514     -170     
==========================================
- Hits         1516     1357     -159     
+ Misses        168      157      -11     

Impacted Files	Coverage Δ
flutter/lib/src/sentry_flutter.dart
flutter/lib/src/file_system_transport.dart
flutter/lib/src/widgets_binding_observer.dart
.../lib/src/navigation/sentry_navigator_observer.dart

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bb04c3d...f66572f. Read the comment docs.

[marandaneto]

every other fixture initializes the fields directly, why is it within a ctor and making them late, is there any benefit in Dart?

[ueman]

I can't initialize Hub in a field because it needs SentryOptions. I think all other properties could be initialized in a field.

[ueman]

I would like to land #413 before this, so I can implement #390 (comment) in this PR.

[marandaneto]

left a suggestion, other than that, LGTM