Our baseline is v5.4 and cgroup v2 is enforced now,
which means CPU accounting is cheap everywhere without
requiring any controller, hence just remove the directive.
Fedora and friends has a drop-in config for the settings in
/usr/lib/systemd/user/slice.d/ . Hence, settings in the main .slice may be
overridden. Let's set below in a drop-in with higher decimal prefix.
Also, rename override.conf -> 99-managed-oom-preference.conf for the same reason.
Having these named differently than the test itself mostly creates
unecessary confusion and makes writing logic against the tests harder
so let's rename the testsuite-xx units and scripts to just use the
test name itself.