Add tests for the Dendrite admin APIs

[S7evinK]

Contains a breaking change, since the endpoints /_dendrite/admin/evacuateRoom/{roomID} and /_dendrite/admin/evacuateUser/{userID} are now using POST instead of GET

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.30 🎉

Comparison is base (234ed60) 65.41% compared to head (4de5c2f) 65.72%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3028      +/-   ##
==========================================
+ Coverage   65.41%   65.72%   +0.30%     
==========================================
  Files         495      495              
  Lines       53623    53606      -17     
==========================================
+ Hits        35078    35233     +155     
+ Misses      14894    14705     -189     
- Partials     3651     3668      +17     

Flag	Coverage Δ
unittests	43.66% <100.00%> (+0.49%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
clientapi/routing/admin.go	60.92% <100.00%> (+30.72%)	⬆️
clientapi/routing/routing.go	58.07% <100.00%> (+0.86%)	⬆️
roomserver/internal/perform/perform_admin.go	28.39% <100.00%> (+18.42%)	⬆️

... and 11 files with indirect coverage changes

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[kegsay]

This is breaking due to it going from GET -> POST, but that is fine. Please mark it as such in this PR / merge commit so we remember though!