Pass test project via --project flag

Microsoft.Testing.Platform's `dotnet test` integration rejects a positional
project argument with the message "Specifying a project for 'dotnet test'
should be via '--project'" and exits 1. The previous form
`dotnet test path/to/test.csproj` therefore failed on every CI platform
without ever invoking the test runner.

Using `--project path/to/test.csproj` makes all 17 tests run and pass
locally with the same Release/--no-build invocation the workflow uses.

https://claude.ai/code/session_017B9mN9F7C3pWGZRyoKBd6R

Author: claude

.github/workflows/cross-platform.yml

@@ -53,4 +53,4 @@ jobs:
         run: dotnet build CredentialCache.sln --configuration Release --no-restore
 
       - name: Test
-        run: dotnet test CredentialCache.Test/CredentialCache.Test.csproj --configuration Release --no-build
+        run: dotnet test --project CredentialCache.Test/CredentialCache.Test.csproj --configuration Release --no-build