build(deps): Bump xunit in /tests/OrasProject.Oras.Tests

Bumps [xunit](https://github.com/xunit/xunit) from 2.6.6 to 2.7.0.
- [Commits](xunit/xunit@2.6.6...2.7.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

tests/OrasProject.Oras.Tests/OrasProject.Oras.Tests.csproj

@@ -15,7 +15,7 @@
     </PackageReference>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
     <PackageReference Include="Moq" Version="4.20.70" />
-    <PackageReference Include="xunit" Version="2.6.6" />
+    <PackageReference Include="xunit" Version="2.7.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.6">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>