ART: Make run-test 931 more defensive wrt/ shutdown

Make the main thread loop a bit longer waiting for the agent
thread to die, so we don't unload under its feet.

Bug: 34701734
Test: m test-art-host-run-test-931-agent-thread
Change-Id: I554d72249c366b17c644f6c17c2b59fda2ab810e
1 file changed