Bump xunit from 2.7.1 to 2.8.0

[dependabot[bot]]

Bumps xunit from 2.7.1 to 2.8.0.

Commits

• be260b3 v2.8.0
• a8ceb66 #783: Add -useansicolor flag to console runner (v2)
• 7b0ff93 Don't show /aggressive with unlimited threads
• 46cdf06 Support parallel algorithm in MSBuild runner
• b4aa876 Support multipler syntax in MSBuild runner
• 6790b48 Add aggressive display to TestFrameworkEnvironment reported by XunitTestAssem...
• 3dd7e91 Update mocks to make CollectionBehaviorAttribute property values optional
• 4c82dea Asking for default threads should set 0, not null
• d73cdef Should not try to use a semaphore when we've been asked for unlimited threads
• 3722e54 Enable multiplier style max threads support
• Additional commits viewable in compare view

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