profiler_test.py fixes and add coverage to Cloud TPU CI
* Add deps to test requirements, including in new
`collect-profile-requirements.txt` (to avoid adding tensorflow to
`test-requirements.txt`).
* Use correct Python executable `ProfilerTest.test_remote_profiler`
(`python` sometimes defaults to python2)
* Run computations for longer in `ProfilerTest.test_remote_profiler`,
othewise `collect_profile` sometimes misses it.
