Add PantsRunIntegrationTest into pantsbuild.pants.testinfra package.

Testing Done:
ran `build-support/bin/release.sh -n` and verified the inclusion.

Bugs closed: 696

Reviewed at https://rbcommons.com/s/twitter/r/1185/
Sync of https://github.com/pantsbuild/pants/commit/44a11e8ffd2a158fcc19095c5834a7e65115b644