ART: More waiting in 063

Wait till the initial reaper thread goes into TIMED_WAITING, in an
effort to make the test more deterministic.

Bug: 2310700
Test: art/test/testrunner/testrunner.py -b --host -t 063
Change-Id: Id4a213a20c21ed0f6a2a73daf1927f7dcf98040d