Upgrade xunit version to avoid specialized restore

Upgrade xunit version to avoid specialized restore

Upgrade xunit from 2.1.0 to 2.2.0-beta2-build3300.

Restore xunit.runner.msbuild during normal test build.

Remove unused reference to xunit 2.1.0.

Replace "2.1.0" with "$(XunitPackageVersion)" for direct path to files.

Upgrade xunit used by GCPerfTestFramework and fix helix path.

Upgrade Microsoft.DotNet.xunit.performance and Microsoft.DotNet.xunit.performance.metrics to 1.0.0-alpha-build0040.

Upgrade XunitPerformanceDependency packages to 1.0.0-alpha-build0040.

Include scripts project.json in validation, update them.

Don't use helix.targets in generated xunit wrappers.

Copy TestTFM defaulting logic from CoreFX dir.props. Tweak default wrapper TFM per platform, and use real RID instead of relying on fallthrough.