build(deps): Bump xunit from 2.6.0 to 2.6.4 (#788)

Bumps [xunit](https://github.com/xunit/xunit) from 2.6.0 to 2.6.4.
- [Commits](xunit/xunit@2.6.0...2.6.4)

---
updated-dependencies:
- dependency-name: xunit
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

testing/ReactiveUI.Samples.Testing.SimpleViewModels.Tests/ReactiveUI.Samples.Testing.SimpleViewModelsUnitTests.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="ReactiveUI.Testing" Version="$(ReactiveUIVersion)" />
-    <PackageReference Include="xunit" Version="2.6.0" />
+    <PackageReference Include="xunit" Version="2.6.4" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\ReactiveUI.Samples.Testing.SimpleViewModels\ReactiveUI.Samples.Testing.SimpleViewModels.csproj" />