test: reenable the unread count test in code coverage

[bnjbvr]

Let's see if the test correctly passes, now that all the racy behaviors should have been fixed with #2978.

Fixes #2963.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b680c50) 83.63% compared to head (0729dfe) 83.68%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3044      +/-   ##
==========================================
+ Coverage   83.63%   83.68%   +0.05%     
==========================================
  Files         222      222              
  Lines       23290    23290              
==========================================
+ Hits        19478    19490      +12     
+ Misses       3812     3800      -12     

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[bnjbvr]

Turns out the real reason why the test wasn't passing was the same root cause as #3031. The client's room member information was missing, meaning we couldn't compute a push context, so we couldn't compute notifications/mentions either. Fortunately, the fix is simple here too. (I don't think this requires another round of review, but wanted to pinpoint the actual fix).