testsuite: don't run grep NUL tests when we know the would fail

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/testsuite/runtest b/testsuite/runtest
index 5c185c5..5ddf07d 100755
--- a/testsuite/runtest
+++ b/testsuite/runtest
@@ -128,7 +128,7 @@
 export OPTIONFLAGS=:$(
 	sed -nr 's/^CONFIG_//p' "$bindir/.config" |
 	sed 's/=.*//' | xargs | sed 's/ /:/g'
-	)
+	):
 
 status=0
 for applet in $applets; do