Bump xunit from 2.7.0 to 2.8.1

[dependabot]

Bumps xunit from 2.7.0 to 2.8.1.

Commits

• ba2ae9b v2.8.1
• 151b8d0 Use 'dotnet format' instead of 'dotnet dotnet-format'
• be6db6f #2931: Tighten up types to prevent accidentically calling AddOrGet with a Con...
• f466d81 #2927: Misleading error message when class used in IClassFixture<> throws exc...
• 1911ea7 Missed unit test updates
• f497d65 Cannot use full assembly path as dictionary key for execution options lookup,...
• c2f2d47 Add ability to provide live output messages from running tests
• 013093b Polyfill in StringSyntaxAttribute
• e1e4c2e #2719: Class with custom Fact with throwing Skip should fail appropriately
• 8b0b13c Clarify naming
• Additional commits viewable in compare view

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