Remove emtpy test file
New rake task for running all tests in parallel

This uses test-queue, which is faster, more
flexible and has less ceremony than parallel_test. Plus parallel_test
only runs unit tests, which lessens it's usefulness somewhat.
Stop DataMigratorTests falling over in parallel

These tests do not pass reliably when run in parallel. I suspect it
has to do with transactions not being rolled back correctly, meaning
the data migration remains in the database as having been run, causing
the tests to fail. Destroying all the data migration records ahead of
time ensures we always start with a clean slate.
Update jenkins-parallel.sh to use test-queue task
Merge pull request #570 from alphagov/parallel-mark-two

Parallel Mk.2