testing/tests/download: test git tag

Add a tag to the static repo and check on the git refs test case the
download of a git package:
 - with the name of a tag as version;
 - with the sha1 of a tag itself as version;
 - with the partial sha1 of a tag itself as version;
 - with the sha1 of a commit pointed by a tag as version;
 - with the partial sha1 of a commit pointed by a tag as version;
 - with the sha1 of a commit reachable only by a tag as version;
 - with the partial sha1 of a commit reachable only by a tag as version.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@datacom.ind.br>
Cc: Arnout Vandecappelle <arnout@mind.be>
Tested-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>