ART: More rework of test 912

Ensure that the JIT finished compiling before enabling events.
Also run and join a thread before the test.

Bug: 31684920
Test: m ART_TEST_JIT=true ART_TEST_RUN_TEST_RELOCATE_NO_PATCHOAT=true test-art-host-run-test-912-classes
Change-Id: I86ce3820923fb6d68a5cb2ccc971fca16fe5e35f
1 file changed