Implement JVMTI can_signal_thread capability.

Implements the JVMTI can_signal_thread capability and all associated
methods and behaviors. This includes both the StopThread and
InterruptThread functions.

This CL contains the tests for the previous CL.

Test: ./test.py --host -j50
Test: stress --cpu 59 && while ./test/run-test --host 1934; do; done

Bug: 62821960
Bug: 34415266
Change-Id: I7b6fc37da0d2673caa993e486f078cf129d74c0f