tests: Cleanup generated files on expected failure in grub_cmd_cryptomount

tests: Cleanup generated files on expected failure in grub_cmd_cryptomount

grub-shell-luks-tester only cleans up generated files when the test it
runs returns success. Sometimes tests are run that should fail. Add
a --xfail argument to grub-shell-luks-tester and pass it from
grub_cmd_cryptomount when invoking a test that is expected to fail.

Reported-by: Thomas Schmitt <scdbackup@gmx.net>
Signed-off-by: Glenn Washburn <development@efficientek.com>
Tested-by: Thomas Schmitt <scdbackup@gmx.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>