Don't report OTHER references in test 913

These references are from vm-internal sources and can change depending
on the configuration and current state of the runtime. Notably
additional roots in the OTHER category will be present if the runtime
is current using instrumentation trampolines for tracing.

Test: ./test/testrunner/testrunner.py --host --debug --runtime-option=-Xplugin:libtracefast-trampolined.so
Change-Id: Ib8c2da7b203edd59be296028557bc074f9d0dceb