Move TestAccessors to extension properties

[JeremyKuhne]

Cleaner syntax. Also update exception handling to be more useful.

This is to better align with what I'm putting in the SDK repo. Hopefully I can eventually move this upstream to some of our shared test infra.

It does make it a little bit more awkward for VB, but not unusable.

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

:x: Patch coverage is 98.16425% with 19 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 97.40559%. Comparing base (ceb6911) to head (a943403). :warning: Report is 5 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main      #14134          +/-   ##
====================================================
+ Coverage   77.15337%   97.40559%   +20.25222%     
====================================================
  Files           3279        1214        -2065     
  Lines         645325      357384      -287941     
  Branches       47720        5597       -42123     
====================================================
- Hits          497890      348112      -149778     
+ Misses        143748        8480      -135268     
+ Partials        3687         792        -2895     

Flag	Coverage Δ
Debug	97.40559% <98.16425%> (+20.25222%)	:arrow_up:
integration	?
production	?
test	97.40559% <98.16425%> (-0.00190%)	:arrow_down:
unit	?

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