selftests: net: give up on the cmsg_time accuracy on slow machines

Commit b9d5f5711dd8 ("selftests: net: increase the delay for relative
cmsg_time.sh test") widened the accepted value range 8x but we still
see flakes (at a rate of around 7%).

Return XFAIL for the most timing sensitive test on slow machines.

Before:

# ./cmsg_time.sh
Case UDPv4 - TXTIME rel returned '8074us - 7397us < 4000', expected 'OK'
FAIL - 1/36 cases failed

After:

# ./cmsg_time.sh
Case UDPv4 - TXTIME rel returned '1123us - 941us < 500', expected 'OK' (XFAIL)
Case UDPv6 - TXTIME rel returned '1227us - 776us < 500', expected 'OK' (XFAIL)
OK

Reviewed-by: Przemek Kitszel <[e-mail]>
Reviewed-by: Willem de Bruijn <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/cmsg_time.sh

···
 TOTAL=0
 
 check_result() {
+    local ret=$1
+    local got=$2
+    local exp=$3
+    local case=$4
+    local xfail=$5
+    local xf=
+    local inc=
+
+    if [ "$xfail" == "xfail" ]; then
+	xf="(XFAIL)"
+	inc=0
+    else
+	inc=1
+    fi
+
     ((TOTAL++))
-    if [ $1 -ne 0 ]; then
-	echo "  Case $4 returned $1, expected 0"
-	((BAD++))
+    if [ $ret -ne 0 ]; then
+	echo "  Case $case returned $ret, expected 0 $xf"
+	((BAD+=inc))
     elif [ "$2" != "$3" ]; then
-	echo "  Case $4 returned '$2', expected '$3'"
-	((BAD++))
+	echo "  Case $case returned '$got', expected '$exp' $xf"
+	((BAD+=inc))
     fi
 }
 
···
 		 awk '/SND/ { if ($3 > 1000) print "OK"; }')
 	check_result $? "$ts" "OK" "$prot - TXTIME abs"
 
-	[ "$KSFT_MACHINE_SLOW" = yes ] && delay=8000 || delay=1000
+	[ "$KSFT_MACHINE_SLOW" = yes ] && xfail=xfail
 
-	ts=$(ip netns exec $NS ./cmsg_sender -p $p $i 1234 -t -d $delay |
+	ts=$(ip netns exec $NS ./cmsg_sender -p $p $i 1234 -t -d 1000 |
 		 awk '/SND/ {snd=$3}
 		      /SCHED/ {sch=$3}
-		      END { if (snd - sch > '$((delay/2))') print "OK";
-			    else print snd, "-", sch, "<", '$((delay/2))'; }')
-	check_result $? "$ts" "OK" "$prot - TXTIME rel"
+		      END { if (snd - sch > 500) print "OK";
+			    else print snd, "-", sch, "<", 500; }')
+	check_result $? "$ts" "OK" "$prot - TXTIME rel" $xfail
     done
 done