Indicate failed imports in list-tests.

Test listing now explicitly indicates by printing 'Failed to import' and
exiting (2) when an import has failed rather than only signalling through the
test name. This plus the introduction of a public helper lays the groundwork
for fixing a usability bug in testrepository caused by the interaction between
failed listings and user test filters.

Change-Id: I7fb8cb70c515c54d8febd68ba791c27db0a7a9c4
Closes-Bug: #1245672