Review tests in psf_functions_test.py

[gonzaponte]

... specifically

• test_add_empty_sensors_and_normalize_q
• test_add_empty_sensors_and_normalize_q_file
• test_hdst_psf_processing

The input data for these tests is not the expected one. We expect hits that correspond to one z-sample and one sensor. Because these tests only check for continuity, we've never noticed they are incorrect.

We should also add more meaningful tests.