test: Make the sysext test to run with --root and without

test: Make the sysext test to run with --root and without

I was bitten several times by testing things only with --root flag, so this
commit prepares the existing test cases to run on / too. This required the test
cases to clean up after themselves, thus I have put each test case in a
separate subshell and used traps to do the cleanups.

I needed to change the hierarchy used by the test extension to /opt, because
unmounting /usr often failed with EBUSY.