We had a unit test that was checking for the mercurial module to be

loaded before attempting to create a Mercurial-based repository. This

check wasn't correct, because we don't care if the module is accessible.

We only care if the hg binary is.
Another unit test wasn't performing a check at all, erroring out instead

when trying to create the repository.
This change fixes both issues by checking for the hg binary to be

available before running the test.

Unit tests pass with and without the hg binary (and without access to

the mercurial module).