Completion integration and perf test in C# + C# Dev Kit

Add a completion perf test to the C# Dev Kit suite - [testPerf.yml - Repos (visualstudio.com)](https://devdiv.visualstudio.com/DevDiv/_git/vs-green?path=/azure-pipelines/testPerf.yml). It would be good to cover 2 things:

1) Expand the existing open solution test to trigger completion as a first invoke of completion
2) Add a new completion test that tests completion after the session settles down