XunitTestFrameworkExecutor has hidden dependency to XunitTestFrameworkDiscoverer

[dainius-bekeris]

Found this hard way: one who wants custom discoverer shall also create specialized executor and override CreateDiscoverer() method. While it's possible to solve it without changes in framework, I think it would be better to change design: make dependency explicit, update XunitTestFramework implementation so that it's enough to override only discoverer creation.

Do you think it's possible to include such change into framework? I can do all needed changes.

[bradwilson]

Transferred to issue #2163