In the case that a test was edited, "make test" would erroneously
attempt to add `test_fname` to the list of files to test...which doesn't
exist in that branch. Instead, it should be adding `edited_fname` in
that case.
This replaces the bash script I originally proposed as a way to safely invoke
unit tests with a xonsh script (we need to eat our own dog food). This also
udpates the documentation.
This makefile makes it easier to run things like unit tests and pylint
against the code. Especially before doing a commit. It also helps ensure
unit tests are not affected by local config files (e.g., ~/.xonshrc).
