Testing: Flag external dependencies

[Lestropie]

Came to mind modifying #2855.

The full set of Python command tests can't always be run; partly because of time, partly because of external dependencies. There's now label pythonci that selects a small set of tests to run, which will eventually be incorporated into CI. What's missing is the ability to run every test except for those that have external dependencies. Should be easy to add a label to tests that invoke anything outside of MRtrix3, eg. "extdep".