common: move SharedPtrRegistry test after t.join

The thread created to test SharedPtrRegistry race conditions updates a
value ( ptr ) that is tested by the main gtest thread but is not
protected by a lock. Instead of adding a lock, the main thread tests
the value after pthread_join() on the child thread.

http://tracker.ceph.com/issues/6130 fixes #6130

Signed-off-by: Loic Dachary <loic@dachary.org>