http-service: Add extra doctest for dashes in _is_valid_module_filename

You can have a python file with dashes in the filename, and you can even
run it with python, but you can't import it with the standard `import
modulename` syntax.

The function already passes the new doctest.
http-service: Cleanup: Same type for both codepaths in service.TestRunner.list_refs

The other codepath sets this variable to a list, so here we should too,
for clarity.
http-service: Add "/v1/refs" endpoint

To find out the branches available in the test-pack git repo, and their
corresponding head sha.

Unfortunately the web front-end can't fetch this information directly
from gitweb (because gitweb doesn't expose the information in an easily
parseable manner), so we need the http-service to provide it.

In future we might want to return the tags, not just the branches.

The change to test-pack.bundle is adding a new branch, for the
self-tests.