Update to xunit v3

[martincostello]

• Update to xunit v3.
• Avoid infinite loop and throw if test times out.

Resolves #2439.

[Copilot]

Copilot reviewed 143 out of 158 changed files in this pull request and generated no comments.

Files not reviewed (15)

• Directory.Packages.props: Language not supported
• eng/Test.targets: Language not supported
• src/Snippets/Snippets.csproj: Language not supported
• test/Polly.Core.Tests/CircuitBreaker/Controller/CircuitStateControllerTests.cs: Evaluated as low risk
• test/Polly.Core.Tests/CircuitBreaker/BrokenCircuitExceptionTests.cs: Evaluated as low risk
• test/Polly.Core.Tests/CircuitBreaker/IsolatedCircuitExceptionTests.cs: Evaluated as low risk
• test/Polly.Core.Tests/Fallback/FallbackHandlerTests.cs: Evaluated as low risk
• test/Polly.Core.Tests/CircuitBreaker/CircuitBreakerResiliencePipelineBuilderTests.cs: Evaluated as low risk
• test/Polly.Core.Tests/CircuitBreaker/CircuitBreakerOptionsTests.cs: Evaluated as low risk
• test/Polly.Core.Tests/CircuitBreaker/CircuitBreakerResilienceStrategyTests.cs: Evaluated as low risk
• test/Polly.Core.Tests/Fallback/FallbackResiliencePipelineBuilderExtensionsTests.cs: Evaluated as low risk
• test/Polly.Core.Tests/CircuitBreaker/CircuitBreakerStateProviderTests.cs: Evaluated as low risk
• src/Snippets/Docs/Testing.cs: Evaluated as low risk
• build.cake: Evaluated as low risk
• test/Polly.Core.Tests/CircuitBreaker/CircuitBreakerPredicateArgumentsTests.cs: Evaluated as low risk

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.12%. Comparing base (6d5fcd6) to head (e28f271).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2440   +/-   ##
=======================================
  Coverage   96.12%   96.12%           
=======================================
  Files         309      309           
  Lines        7118     7118           
  Branches     1008     1008           
=======================================
  Hits         6842     6842           
  Misses        222      222           
  Partials       54       54           

Flag	Coverage Δ
linux	96.12% <ø> (ø)
macos	96.12% <ø> (ø)
windows	96.11% <ø> (ø)

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