oeqa/selftest/cases/devtool.py: skip all tests require folder a git repo

oeqa/selftest/cases/devtool.py: skip all tests require folder a git repo

Devtool selftests require poky dir a git repo, when downloading poky as a tar,
this is not the case. Those tests will now skipped.

[YOCTO #12389]

Signed-off-by: Thomas Roos <throos@amazon.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 95a5bc130dc51ea9de95c64dbf0e9c7892415d50)
Signed-off-by: Steve Sakoman <steve@sakoman.com>