test_sys_checkout: use actual paths in on-the-fly configs rather than env var

[johnpaulalex]

This will make it easier to test (in the near future) that checkout_externals actually checked out the desired repo dir.

The env-var path is still tested by two tests which rely on a checked-in externals.cfg that relies on this same environment variable (MANIC_TEST_BARE_REPO_ROOT)

More clearly document that env vars are passed to checkout_externals in the tests via os.environ, and that the command line assembled in _execute_checkout_in_dir is just for manual reproducibility.

User interface changes?: No

Fixes: None

Testing:
test removed: none
unit tests: none
system tests: 'make stest' passes
manual testing: none

[billsacks]

I like your removal of environment variables where they aren't needed!

I think it would be a bit better to keep the deletion of os.environ['MANIC_TEST_BARE_REPO_ROOT'] in the teardown function. The benefit of doing that is that the teardown function is called even if the test raises an exception and so bails early. Now I guess you'd need to do this conditionally, like:

if 'MANIC_TEST_BARE_REPO_ROOT' in os.environ:
    del os.environ['MANIC_TEST_BARE_REPO_ROOT']

But I don't feel this is critical if you prefer to leave things as they are.

[johnpaulalex]

sounds good, I added another commit to do this.

[billsacks]

Looks great, thanks!