Remove the concept of 'integration tests' from test-webkitpy.

There's not really a useful difference between unittests and integration
tests in webkitpy these days, so this removes the integrationtest concept
and renames the files for consistency w/ everything else.

This is being done in preparation for replacing test-webkitpy with the
generic python test runner (by removing nonstandard things from test-webkitpy).

This also removes the custom TestLoader I wrote since we don't need it any more.

R=eseidel@chromium.org
BUG=402172

Review URL: https://codereview.chromium.org/454223003

git-svn-id: svn://svn.chromium.org/blink/trunk@180235 bbb929c8-8fbe-4397-9dbb-9b2b20218538