libswiftnav-python: improve test coverage on the Python bindings

[mookerji]

The Python bindings now have some test coverage, but we likely want to dramatically improve what we currently have, particularly around DGNSS functionality, the ambiguity test, and tracking: all of which are consumed by various downstream projects (gnss-analysis and peregrine). A end-to-end sanity test would help inspire a little confidence that these continue to work, and also illuminate various C extension/Python compatibility issues that no doubt still exist.