ART: Do not interact with thread object in test 913

The current thread can be reachable from the test's class object.
Ignore it when following references.

Bug: 119942670
Test: m test-art-host
Test: atst CtsJvmtiRunTest913HostTestCases
Change-Id: Ibb9df47010e2432057acc153145ca1a8ed3695d5