test/rilmodem: add new SIM functional tests

This commit adds Python unittest-based functional test scripts
used to test the SIM functionality of an instance of ofono using
the rilmodem/mtkmodem drivers.

These tests replicate the first four functional tests cases in the SIM sub-section of the ofono test plan:

https://wiki.ubuntu.com/Process/Merges/TestPlans/ofono/SIM

These scripts are self-documenting.  Use pydoc, or read the source directly.  Once we build more tests, this internal documentation can be converted to HTML and linked to the wiki for reference.