Enable dx tests to run in parallel by default

Changes run-all-tests to launch tests in parallel by default.

Adds flags:
  --seq         to run tests sequentially.
  --skip <test> to skip specific tests.

Launching tests in parallel and skipping 123-merge-stress enables
a 30s test run on a Z620 compared to 400s running sequentially and
without skipping 123-merge-stress.

Test: dalvik/dx/tests/run-all-tests
Change-Id: I45503c3fe9372dbda9a473ee121b89bff9708b31