temporarily disables run/reflection-sync-subtypes

This test has been a source of spurious failures as in, for example
https://github.com/scala/scala/pull/3029#issuecomment-26811129,
so I'm disabling it for the time being while I investigate the issue.
Merge pull request #3071 from xeno-by/topic/temporarily-disable-sync-tests

temporarily disables run/reflection-sync-subtypes