oeqa/selftest: introduce gitarchive tests

oeqa/selftest: introduce gitarchive tests

Add a test suite for gitarchive.py. For now, only introduce tests on
methods which needs to read existing tags

The tests rely on tmpdirs to create local, "fake" results repository in
order to allow basic git commands

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>