applied Martin Kühl's inverse matching flag to stest

stest.1

@@ -67,6 +67,9 @@ Test that files are not empty.
 .B \-u
 Test that files have their set-user-ID flag set.
 .TP
+.B \-v
+Invert the sense of tests, only failing files pass.
+.TP
 .B \-w
 Test that files are writable.
 .TP