testsuite: Fix line count test

a substring match is not enough, ex: 10 != 1

Fixes: 30383b074d ("tests: Add output testing")
Signed-off-by: Benjamin Poirier <bpoirier@cumulusnetworks.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

testsuite/lib/generic.sh

@@ -121,7 +121,7 @@ test_on_not()
 test_lines_count()
 {
 	echo -n "test on lines count ($1): "
-	if cat "$STD_OUT" | wc -l | grep -q "$1"
+	if [ $(cat "$STD_OUT" | wc -l) -eq "$1" ]
 	then
 		pr_success
 	else