Avoid "async void" tests (dotnet/corefx#39613)

Avoid "async void" tests (dotnet/corefx#39613)

Most of these are just because "async void" tests are rare and require special handling in xunit we needn't use.  But a few of these are plain wrong and were resulting in our not actually testing what we hoped to test.

Commit migrated from https://github.com/dotnet/corefx/commit/50d8c71bb6139f3c06dd820d6d757a095a9fe994