Experimental fsverity.test related GA CI improvements

This does not make fsverity.test working on GA CI, though.

- `--device /dev/loop-control' is required for losetup(8) to work.
- `--privileged' is required foo mount(8) to work, and this makes
  `--security-opt seccomp=unconfined' redundant.
- GA container does not have `/sys/kernel/security' mounted which is
  needed for `/sys/kernel/security/integrity/ima/policy'.
- Enable `set -x` in CI as the logs is everything we have to analyze on
  failures.

Update: with these changes and the UML kernel support, the fsverity.test
is working properly.

Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
[zohar@linux.ibm.com: updated patch description]
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

tests/fsverity.test

@@ -78,7 +78,7 @@ mount_loopback_file() {
 		exit "$FAIL"
 	fi
 
-	mount -o loop ${TST_IMG} $TST_MNT
+	mount -v -o loop ${TST_IMG} $TST_MNT
 	ret=$?
 
 	if [ "${ret}" -eq 0 ]; then