The tests were waiting for the test group to finish before
they could continue with the next test group. Now "core"
and "all" targets directly depend on individual test cases,
allowing parallel Make to get to the next tests quicker and
removing 1/3rd of the total run time.

  make check -j8 -k  5790,16s user 1207,08s system 627% cpu 18:35,49 total
  make check -j8 -k  6250,13s user 1326,77s system 972% cpu 12:59,16 total

Getting:

    Nothing to be done for `ct'

Instead of, in later versions:

    Nothing to be done for 'ct'

Only check for the start of the sentence instead.

And use this in the tests to fetch Rebar only once and then
clone from the local repository. This should speed up tests
a little.

I've reworked how the multi-apps projects are built. In particular
I've made sure the test build is made from the top-level once, and
then only tests are run on this build. It used to build multiple
times and some builds would not include test mode, not good. I've
also fixed issues with running tests in parallel.

All tests now pass with -j8 on my machine. It's possible more
issues remain that are not covered by tests yet though.

Also don't run eunit if there's no src/test directory.