Bump xunit from 2.8.1 to 2.9.0 in /src

[dependabot[bot]]

Bumps xunit from 2.8.1 to 2.9.0.

Commits

• 593e573 v2.9.0
• f070472 Enable XUNIT_HIDE_PASSING_OUTPUT_DIAGNOSTICS environment variable to skip sho...
• 80a2294 For Assert.Equivalent, exclude properties (or property getters) which are obs...
• b6dabde #2955: Ensure minimum thread pool size (v2)
• 5fe01ff #2922: Remove obsolete AssemblyRunner.Start() overloads (v2)
• beab8d9 #1916: Don't try to serialize generic argument types (v2)
• 27c70a7 Restore the Func<TestOutputHelper> so we re-create the output helper for each...
• 1fac6f8 #2377: Test logs get mixed across test cases
• e022e5c Unit test for #2271 (v2)
• 21f6aee Re-enable IDE0290 as we're using 'dotnet format' now
• Additional commits viewable in compare view

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