run-tests with jvmti-stress configuration

Adds a jvmti-stress mode to our run-tests. This mode runs our test
suite with every class-load being intercepted and undergoing
redefinition.

Test: DEXTER_BINARY="/path/to/built/dexter" \
      ./testrunner/testrunner.py --host --jvmti-stress -j40

Change-Id: I898e5cffea42a203aa4c45981b48baf8dc64fbb6