dx: Throttle run-all-tests script

Use file locks to limit the load of the run-all-tests script.

Propagate terminate request to child processes when SIGINT is
received.

Test: . build/envsetup.sh; lunch; art/tools/buildbot-build.sh --host -j32; dalvik/dx/tests/run-all-tests
Bug: 70525148
Change-Id: Ibedfd06786505f1e93152cc694d8cb3c1db17311