selftests: net: avoid just another constant wait

Using hard-coded constant timeout to wait for some expected
event is deemed to fail sooner or later, especially in slow
env.

Our CI has spotted another of such race:
   # TEST: ipv6: cleanup of cached exceptions - nexthop objects          [FAIL]
   #   can't delete veth device in a timely manner, PMTU dst likely leaked

Replace the crude sleep with a loop looking for the expected condition
at low interval for a much longer range.

Fixes: b3cc4f8a ("selftests: pmtu: add explicit tests for PMTU exceptions cleanup")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/fd5c745e9bb665b724473af6a9373a8c2a62b247.1706812005.git.pabeni@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/pmtu.sh

@@ -1957,6 +1957,13 @@ check_command() {
 	return 0
 }
 
+check_running() {
+	pid=${1}
+	cmd=${2}
+
+	[ "$(cat /proc/${pid}/cmdline 2>/dev/null | tr -d '\0')" = "{cmd}" ]
+}
+
 test_cleanup_vxlanX_exception() {
 	outer="${1}"
 	encap="vxlan"
@@ -1987,11 +1994,12 @@ test_cleanup_vxlanX_exception() {
 
 	${ns_a} ip link del dev veth_A-R1 &
 	iplink_pid=$!
-	sleep 1
-	if [ "$(cat /proc/${iplink_pid}/cmdline 2>/dev/null | tr -d '\0')" = "iplinkdeldevveth_A-R1" ]; then
+	for i in $(seq 1 20); do
+		check_running ${iplink_pid} "iplinkdeldevveth_A-R1" || return 0
+		sleep 0.1
+	done
 	err "  can't delete veth device in a timely manner, PMTU dst likely leaked"
 	return 1
-	fi
 }
 
 test_cleanup_ipv6_exception() {