Fix decoration of TestConcurrentEvents

TestConcurrentEvents was marked with a XFAIL decorator at class level, which actually does not
work, and causes the class to be silently skipped everywhere. It seems that making it work at
class level is quite a difficult task, so I will just move it to the individual test methods. I
will follow this up with a commit which makes the decorator blow up in case someone tries to
apply it to a class in the future.

llvm-svn: 257901