Ensure that every test begins with a properly reset build root.

Review Request #2165 — Created May 5, 2015 and discarded — Latest diff uploaded

davidt, patricklaw, zundel
This is very important: A failure in any test subclass's setUp()
method would cause the unittest framework not to run tearDown().
And BaseTest strongly assumes that tearDown() always runs in order
to clean up state that must not persist across tests.

Chief among this state is the buildroot. If setUp() crashes after
setting the buildroot to the tmpdir, then tearDown() won't run,
and the next test will see the previous test's tmpdir as its
real buildroot. And because we compute relative paths against that
buildroot, we can in some cases end up with a path relative to the
bad buildroot pointing into the real buildroot! If you've ever seen
files mysteriously deleted after a test failure, and had to re-check
 them out of git, this is why.

Because this is so dangerous, we aggressively crash the entire test
run if we detect this state.

Note that we use the build root setting as a proxy for all the state
that's set up in setUp() and reset in tearDown(), because it's by far
the most important such state.

In the future we hope to get rid of the need to know the real buildroot,
but for now it's vital not to misdetect it.

CI in flight: https://travis-ci.org/pantsbuild/pants/builds/61385432