A fast test-running mode for python tests.

Runs all tests in a single chroot instead of creating one per test target.

Benefits:
  1) The pants tests run in 12 seconds instead of 160 seconds.
  2) Test isolation can actually hide problems sometimes.

Drawbacks:
  Individual chroots are better for verifying that a test
  target's dependencies are self-contained, i.e., that they
  reflect all the actual deps. When running multiple tests
  in a single chroot, test B could provide a dep that test A
  relies on but doesn't declare, thus hiding a missing dep
  problem.

This drawback is minor - it only verifies that the test target's
deps are good, not the deps of the target being tested.