Download tests and tutorial data via python as needed (not in build cmake script)

[alecjacobson]

#164 added a command line arg so that tests/test_basic.py could be run in a generic environment (removing old hard-coded paths). Unfortunately, this clashes with the unittest modules arg parsing (e.g., for running a single specified test). This moves that parameter to an environment variable.

[alecjacobson]

tests should be runnable in a generic environment (this is also what CIBW_TEST_COMMAND expects). Currently the build cmake scripts are downloading the test data, even though the build is not using it. This PR is trying to fix atop that mistake. If the latest attempt works, I'll merge it and move on. If it doesn't, a more dramatic and superior fix would be for the test (and tutorial) to download their own data.