Almost all tests were manually mounting/unmounting $TESTDIR/root
from the loopback image; this moves all that into test-functions
so the test setup functions are simplier.
Also add test_setup_cleanup() function, to cleanup what is mounted
by create_empty_image_rootdir()
The `set -e` option is incompatible with a subshell/compound command,
which is followed by || <EXPR>. In such case, the -e option is ignored
in all affected subshells/functions (see man bash(1) for command `set`).
