Being able to see which tests have been skipped is crucial to confirming your desired tests have been skipped when skimming down the list of tests that have passed.

This PR implements skip_test that can be used to skip a test and show the test has been skipped in the output:

[...]
test: ghe-restore with current backup leaked key detection ...     OK
test: ghe-restore fails when restore to an active HA pair ...      SKIPPED
test: ghe-restore honours --version flag ...                       OK
[...]

Update: I've just discovered that when there's a failure building the Debian pkg, we display the results of building the tarball and not the Debian pkg. This PR fixes that too.

/cc @github/backup-utils