Bump xunit from 2.4.2 to 2.6.4 (#26)

Bumps [xunit](https://github.com/xunit/xunit) from 2.4.2 to 2.6.4.
- [Commits](https://github.com/xunit/xunit/compare/2.4.2...2.6.4)

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

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

AtomicLibTests/AtomicLibTests.csproj

@@ -10,7 +10,7 @@
 
     <ItemGroup>
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
-        <PackageReference Include="xunit" Version="2.4.2" />
+        <PackageReference Include="xunit" Version="2.6.4" />
         <PackageReference Include="xunit.runner.visualstudio" Version="2.5.6">
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
             <PrivateAssets>all</PrivateAssets>