Write new tests for functions.py

[aaarendt]

The tests in test_functions.py rely on the test database infrastructure that is no longer supported with our factory approach. All files and classes using the old approach are proposed to be deleted in #170. Note that these tests were already being skipped in the test suite.

This issue tracks conversations on how to write tests for the POSTGIS functions that manipulate rasters in functions.py, and how to handle testing of rasters using our new approach.