Add unit tests on TimelineItemsSubscriber

[bmarty]

Content

Add unit tests on TimelineItemsSubscriber, in particular, cover the fix done in #3538.

Motivation and context

Improve code coverage.

Screenshots / GIFs

Tests

• Should not impact production code.

Tested devices

• [ ] Physical
• [ ] Emulator
• OS version(s):

Checklist

• [ ] Changes have been tested on an Android device or Android emulator with API 23
• [ ] UI change has been tested on both light and dark themes
• [ ] Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• [ ] Pull request is based on the develop branch
• [ ] Pull request title will be used in the release note, it clearly define what will change for the user
• [ ] Pull request includes screenshots or videos if containing UI changes
• [ ] Pull request includes a sign off
• [ ] You've made a self review of your PR

[github-actions[bot]]

:iphone: Scan the QR code below to install the build (arm64 only) for this PR. If you can't scan the QR code you can install the build via this link: https://i.diawi.com/nSPFKY

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.68%. Comparing base (e21c8d1) to head (f89463c). Report is 2 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #3554 +/- ## ======================================== Coverage 82.67% 82.68% ======================================== Files 1732 1732 Lines 40970 40970 Branches 4962 4962 ======================================== + Hits 33873 33874 +1 Misses 5341 5341 + Partials 1756 1755 -1 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud