Enable testing of pre-built binaries.

[wearhere]

To support testing release candidates, where the candidate binary would be built before testing and then submitted without re-building if the tests passed.

Implementation notes

A new APP argument should be added to subliminal-test. subliminal-test should require one of PROJECT, WORKSPACE, or APP be specified. The documentation should say that SIM_DEVICE or HW_ID should be provided depending on whether APP is built for simulator or device. [The use case for this feature is to test device builds, but I don't see a need to enforce that. --Jeff]

If APP is provided, subliminal-test should skip building the app (everything in the "### Build app" section). subliminal-test should also not attempt to rebuild the app if instruments fails (search for "build_app").