refactor(test): reorganize test files to mirror lib/src structure (#140)

## Description

Reorganizes the test directory structure to mirror the `lib/src/` folder
layout, making it easier to find and maintain tests for specific
modules.

### Changes

Test files have been moved/renamed to match their corresponding source
files:

| Before | After |
|--------|-------|
| `test/rum_flutter_test.dart` | `test/src/faro_test.dart` |
| `test/data_collection_policy_test.dart` |
`test/src/data_collection_policy_test.dart` |
| `test/faro_session_attributes_test.dart` |
`test/src/faro_session_attributes_test.dart` |
| `test/unit_test/batch_transport_test.dart` |
`test/src/transport/batch_transport_test.dart` |
| `test/payload_extension_test.dart` |
`test/src/util/payload_extension_test.dart` |
| `test/timestamp_extension_test.dart` |
`test/src/util/timestamp_extension_test.dart` |
| Integration tests | `test/src/integrations/` |

## Related Issue(s)

N/A - Internal code organization improvement

## Type of Change

- [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
- [ ] 🚀 New feature (non-breaking change which adds functionality)
- [ ] 💥 Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] 📝 Documentation
- [ ] 📈 Performance improvement
- [x] 🏗️ Code refactoring
- [ ] 🧹 Chore / Housekeeping

## Checklist

- [x] I have made corresponding changes to the documentation
- [x] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the CHANGELOG.md under the "Unreleased" section

## Additional Notes

This is a pure structural refactor - no test logic was changed, only
file locations. This aligns test organization with the SDK's source
structure for better maintainability.

Made with [Cursor](https://cursor.com)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Test-only change adjusting an import path; no production code or logic
is modified, so risk is limited to potential test compilation issues.
> 
> **Overview**
> Updates `test/src/integrations/sampling_integration_test.dart` to
import `FakeRandomValueProvider` from its new location under
`test/helpers/` (via `../../helpers/...`) instead of the old relative
path, aligning the test with the reorganized test directory structure.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
12f35c2. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: robert-northmind

…it_test/data_collection_policy_test.dart test/src/data_collection_policy_test.darttest/unit_test/data_collection_policy_test.dart renamed to test/src/data_collection_policy_test.dart test/unit_test/data_collection_policy_test.dart renamed to test/src/data_collection_policy_test.dart

@@ -14,7 +14,7 @@ import 'package:mocktail/mocktail.dart';
 import 'package:package_info_plus/package_info_plus.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
-import '../helpers/fake_random_value_provider.dart';
+import '../../helpers/fake_random_value_provider.dart';
 
 class MockFaroTransport extends Mock implements FaroTransport {}