Add unit tests for the timeline

[jplatte]

So far we've only had integration tests that use a mocked Matrix server. These unit tests don't need that and are thus much easier to write. Of course we should still have some integration tests covering server interaction and the public API, but I think this is the much more realistic way of getting good coverage of all the edge cases of the timeline implementation.

Part of #1103

[codecov]

Codecov Report

Base: 73.58% // Head: 73.69% // Increases project coverage by +0.10% 🎉

Coverage data is based on head (c8bb077) compared to base (93be96c).
Patch coverage: 83.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1192      +/-   ##
==========================================
+ Coverage   73.58%   73.69%   +0.10%     
==========================================
  Files         110      110              
  Lines       12605    12611       +6     
==========================================
+ Hits         9276     9294      +18     
+ Misses       3329     3317      -12     

Impacted Files	Coverage Δ
crates/matrix-sdk/src/room/timeline/mod.rs	82.71% <ø> (ø)
crates/matrix-sdk/src/room/timeline/event_item.rs	72.36% <83.33%> (+6.65%)	⬆️
...ates/matrix-sdk/src/room/timeline/event_handler.rs	72.68% <0.00%> (+3.96%)	⬆️

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