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

[mikadumont]

Add a completion perf test to the C# Dev Kit suite - testPerf.yml - Repos (visualstudio.com). 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