rauc/tests: move some test boilerplate to suite level

The rauc system configuration file is now centrally managed by the suite
prepare logic. The same goes into A and B slots. There is still some
apparent duplication in how /proc/cmdline is handled and how the update
bundle is prepared but I didn't touch those yet, mainly because I don't
know if there is enough common part in the parts that are coming in the
future.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>