Add pushrules tests

[S7evinK]

partly takes care of #2870 by making sure that rule IDs don't start with a dot.

[codecov]

Codecov Report

Patch coverage: 82.75% and project coverage change: +0.02 🎉

Comparison is base (94e81cc) 66.62% compared to head (cb9f94e) 66.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3044      +/-   ##
==========================================
+ Coverage   66.62%   66.65%   +0.02%     
==========================================
  Files         495      497       +2     
  Lines       53625    53621       -4     
==========================================
+ Hits        35729    35739      +10     
- Misses      14257    14303      +46     
+ Partials     3639     3579      -60     

Flag	Coverage Δ
unittests	46.84% <82.75%> (+0.52%)	⬆️

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Impacted Files	Coverage Δ
userapi/api/api.go	70.90% <ø> (ø)
clientapi/routing/pushrules.go	79.03% <78.94%> (+15.87%)	⬆️
userapi/internal/user_api.go	63.12% <85.71%> (+0.41%)	⬆️
internal/pushrules/validate.go	100.00% <100.00%> (ø)

... and 97 files with indirect coverage changes

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