support/testsuite: de-duplicate the systemd runtime tests

The first three lines of all systemd runtime tests are identical, and
they already call into a common function. Therefore, move those lines
into the common function as well.

We need to pass an additional argument for the rootfs type. This changes
the signature, which could create confustion with
InitSystemBase.check_init() that has a different signature. Therefore,
rename the function to check_systemd(). That also allows us to call
self.check_init() directly instead of going through super().

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>