task(tests): mocking, continued

[bnjbvr]

A few new shiny things for the mocking test helpers:

• reuse more code by introducing a generic parameter for the endpoint specific data
• add a MockClientBuilder to help creating a Client already attached to the server, and logged in,
• use Matrix v1.12 for all the testing endpoints, and update mock path matchers

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 6 lines in your changes missing coverage. Please review.

Project coverage is 84.89%. Comparing base (982c6ea) to head (3a9fb22).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/test_utils/mocks.rs	91.66%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4233      +/-   ##
==========================================
- Coverage   84.91%   84.89%   -0.03%     
==========================================
  Files         274      274              
  Lines       29745    29761      +16     
==========================================
+ Hits        25258    25265       +7     
- Misses       4487     4496       +9     

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[poljar]

Looks good.