Improve python CI robustness.

Ensure all tests/python targets are selected when dividing up unit
and integration tests - no longer use aggregator targets as a guide.

This change leaves the aggregators in-place and just divorces CI from
them to ensure we always CI all test targets.

 build-support/bin/ci.sh | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

Testing Done:
Locally:
$ ./build-support/bin/ci.sh

Travis went green here: https://travis-ci.org/pantsbuild/pants/builds/35691561

Bugs closed: 586

Reviewed at https://rbcommons.com/s/twitter/r/1050/