dx: Test enhancements

Add missing file for source debug extension test.

Better filtering of inputs for 127-merge-stress test to avoid
non-deterministic output from run script, no more "Entry not found".

Run merge tests in parallel to reduce test execution time by 50% when
run locally.

Bug: 63131218
Test: dalvik/dx/tests/run-all-tests
Change-Id: I6558fe9cdf0d4d49ee56c20aeb62ea409dd0bcf2