sort: -z outputs NUL terminated lines. Closes bug 1591.

diff --git a/testsuite/runtest b/testsuite/runtest
index 93d5ed6..fc8392a 100755
--- a/testsuite/runtest
+++ b/testsuite/runtest
@@ -1,5 +1,8 @@
 #!/bin/sh
 
+# Usage:
+# runtest [applet1] [applet2...]
+
 # Run one old-style test.
 # Tests are stored in applet/testcase shell scripts.
 # They are run using "sh -x -e applet/testcase".