Log Message:
-----------
tests: mount_injection: ensure cleanup on test failure
The mount_injection test was exiting immediately on failure without
calling lxc_teardown_shmount(), leaving /tmp/mount_injection_test
mounted and causing "Device or resource busy" errors when trying to
remove it.
Store the test result and ensure lxc_teardown_shmount() is always
called before exiting, even when tests fail.