dx: update run-test script and expectations

Update the run-test script to use the javac from prebuilts.

Update expectations for tests that were failing locally and not on the
buildbots. It looks like these failures stem from different javac
versions.

Fix array bounds error in 127-merge-stress.

Bug: 64707326
Test: art/tools/buildbot-build.sh --host && dalvik/dx/tests/run-all-tests

Change-Id: Ibc4b9216db9fed42aee8f239775ee4cb94536b74