Tidy up code in Polly.Specs

[martincostello]

Tidies up various bits of code in Polly.Specs.

• Suppress CA1062, SA1204 and SA1600 warnings centrally for test projects.
• Suppress S104 and S6966 in editorconfig.
• Suppress the CA1030, CA2008 and CA2201 warnings in Polly.Specs.
• Suppress CA1031 at the two usages and re-enable.
• Apply various IDE suggestions to simplify/modernise code.

Contributes to #1290.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.35%. Comparing base (bf6bdb9) to head (697d280).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2466   +/-   ##
=======================================
  Coverage   85.35%   85.35%           
=======================================
  Files         312      312           
  Lines        7464     7464           
  Branches     1121     1121           
=======================================
  Hits         6371     6371           
  Misses        908      908           
  Partials      185      185           

Flag	Coverage Δ
linux	85.35% <ø> (ø)
macos	85.35% <ø> (ø)
windows	85.32% <ø> (ø)

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