testing/tests/download: test git special ref

Add a special ref to the static repo and check on the git refs test case
the download of a git package:
 - with the sha1 of a special ref as version;

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@datacom.ind.br>
Cc: Arnout Vandecappelle <arnout@mind.be>
[Arnout: change to use the sha1 of a special ref instead of the name]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>