Highlight C# code in unit tests

[JakeYallop]

Adds [StringSyntax("C#-test")] to verifier classes to enable syntax highlighting for embedded C# code in visual studio.

Before:

After:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.49%. Comparing base (43709af) to head (26e5bd1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7335   +/-   ##
=======================================
  Coverage   96.49%   96.49%           
=======================================
  Files        1443     1443           
  Lines      345799   345799           
  Branches    11374    11374           
=======================================
+ Hits       333665   333675   +10     
+ Misses       9251     9242    -9     
+ Partials     2883     2882    -1     

[drewnoakes]

LGTM. Verified this works in VS2022 17.11.

Will let @mavasani and @sharwell weigh in on the change though.