Add hooks to avoid collecting code for run-tests.

ensureJitCompiled ensures a method gets compiled, but does
not prevent it from being GC'ed. Add a hammer hook to avoid
doing any JIT code cache GC.

test: test-art-host with gcstress and jit.
Change-Id: I3881f10f454ded9c7a3b385d326c2e08932d39bf