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]

📱 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.68%. Comparing base (e21c8d1) to head (f89463c).
Report is 35 commits behind head on develop.

Additional details and impacted files

@@           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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sonarcloud]

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