tests: skip test_get_process_cmdline_harder if `mount --make-rslave /` fails with EPERM or EACCESS

That call to mount was added as a safeguard against a kernel bug which was fixed in
torvalds/linux@bbd5192.

In principle, the error could be ignored because

* normally everything mounted on /proc/PID should disappear as soon as the PID has gone away
* test-mount-util that had been confused by those phantom entries in /proc/self/mountinfo was
  taught to ignore them in 112cc3b5.

On the other hand, in practice, if the mount fails, then the next one is extremely unlikely to
succeed, so it seems to be reasonable to just skip the rest of `test_get_process_cmdline_harder`
if that happens.

Closes https://github.com/systemd/systemd/issues/9649.