[PATCH iproute2 2/3] testsuite: fix test_on_not on multi-line output

[Dave Seddon <dave.seddon.ca@gmail.com>]

grep -vqE succeeds when any line does not match, even if another line
contains the forbidden pattern. It also fails on empty output.

Accept only exit status 1 from a non-inverted grep: the pattern was
not found. A matching line or a grep error must fail the assertion.

Assisted-by: LLM
Signed-off-by: Dave Seddon <dave.seddon.ca@gmail.com>
---
 testsuite/lib/generic.sh | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/testsuite/lib/generic.sh b/testsuite/lib/generic.sh
index 8b339ec1..ac48f023 100644
--- a/testsuite/lib/generic.sh
+++ b/testsuite/lib/generic.sh
@@ -113,8 +113,9 @@ test_on()
 
 test_on_not()
 {
-	echo -n "test on: \"$1\""
-	if cat "$STD_OUT" | grep -vqE "$1"
+	echo -n "test on absence: \"$1\""
+	grep -qE -- "$1" "$STD_OUT"
+	if [ "$?" -eq 1 ]
 	then
 		pr_success
 	else
-- 
2.55.0