chore: Refactor test related project references

[filzrev]

This PR contains following changes.

1. Move Docfx.Tests.Common project reference to Directory.Build.props level.
   (To suppress diffs when adding shared test code on other PR)
2. Add System.Collections.Immutable/System.Text.Json package references to avoid MSB3277 warnings.
3. Remove unused project references.
4. cherry pick dotnet format commit (4a7ef1a)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.34%. Comparing base (fe673ec) to head (6244ae7).
Report is 499 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10488      +/-   ##
==========================================
+ Coverage   74.31%   79.34%   +5.02%     
==========================================
  Files         536      547      +11     
  Lines       23189    23629     +440     
  Branches     4056     4062       +6     
==========================================
+ Hits        17234    18749    +1515     
+ Misses       4853     3710    -1143     
- Partials     1102     1170      +68     

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