Deduplicate test discovery logic between Common and Logging

[Tratcher]

https://github.com/aspnet/Logging/pull/798/files#diff-4b2f9320dbf4947f20507c2b2a94118d duplicated a lot of logic from Common's Testing package. We assumed they wouldn't change, but then they did:
E.g. aspnet/Logging#832

[pakrym]

@Tratcher or @JunTaoLuo can you take this, please.

[analogrelay]

Closing as this issue is out-of-date and is tracking work that is no longer needed.