test/TEST-25: remove all images at the end of the test

scratch5 would be left behind, and trying to run the test again on the same
image would fail.

test/units/testsuite-25.sh

@@ -133,6 +133,11 @@ rm -rf /var/tmp/extract
 
 rm -rf /var/tmp/scratch
 
+# Test removal
+machinectl remove scratch5
+! test -f /var/lib/machines/scratch5
+! machinectl image-status scratch5
+
 echo OK > /testok
 
 exit 0