[release/9.0] Add .slnx fallback for TestHost content root discovery#64953

wtgodbe merged 2 commits intorelease/9.0dotnet/aspnetcore:release/9.0from

copilot/update-webhostbuilderextensions-behaviordotnet/aspnetcore:copilot/update-webhostbuilderextensions-behaviorCopy head branch name to clipboard