Bump the test-dependencies group with 2 updates (#793)

Bumps the test-dependencies group with 2 updates: [NUnit](https://github.com/nunit/nunit) and [Verify.NUnit](https://github.com/VerifyTests/Verify).


Updates `NUnit` from 4.2.2 to 4.2.2
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/main/CHANGES.md)
- [Commits](nunit/nunit@4.2.2...4.2.2)

Updates `Verify.NUnit` from 28.2.1 to 28.3.1
- [Release notes](https://github.com/VerifyTests/Verify/releases)
- [Commits](VerifyTests/Verify@28.2.1...28.3.1)

---
updated-dependencies:
- dependency-name: NUnit
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: test-dependencies
- dependency-name: Verify.NUnit
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: test-dependencies
...

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

NGitLab.Mock.Tests/NGitLab.Mock.Tests.csproj

@@ -15,6 +15,6 @@
     </PackageReference>
     <PackageReference Include="NUnit3TestAdapter" Version="4.6.0" />
     <PackageReference Include="FluentAssertions" Version="6.12.2" />
-    <PackageReference Include="Verify.NUnit" Version="28.2.1" />
+    <PackageReference Include="Verify.NUnit" Version="28.3.1" />
   </ItemGroup>
 </Project>