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

Bumps [coverlet.msbuild](https://github.com/coverlet-coverage/coverlet) from 6.0.2 to 6.0.3.
- [Release notes](https://github.com/coverlet-coverage/coverlet/releases)
- [Commits](coverlet-coverage/coverlet@v6.0.2...v6.0.3)

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

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

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

@@ -12,7 +12,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="coverlet.msbuild" Version="6.0.2">
+    <PackageReference Include="coverlet.msbuild" Version="6.0.3">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>