selftests: net: cope with slow env in so_txtime.sh test

The mentioned test is failing in slow environments:

  # SO_TXTIME ipv4 clock monotonic
  # ./so_txtime: recv: timeout: Resource temporarily unavailable
  not ok 1 selftests: net: so_txtime.sh # exit=1

Tuning the tolerance in the test binary is error-prone and doomed
to failures is slow-enough environment.

Just resort to suppress any error in such cases. Note to suppress
them we need first to refactor a bit the code moving it to explicit
error handling.

Fixes: af5136f ("selftests/net: SO_TXTIME with ETF and FQ")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/2142d9ed4b5c5aa07dd1b455779625d91b175373.1707730902.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

tools/testing/selftests/net/so_txtime.sh

@@ -5,6 +5,7 @@
 
 set -e
 
+readonly ksft_skip=4
 readonly DEV="veth0"
 readonly BIN="./so_txtime"
 
@@ -46,7 +47,7 @@ ip -netns "${NS2}" addr add 192.168.1.2/24 dev "${DEV}"
 ip -netns "${NS1}" addr add       fd::1/64 dev "${DEV}" nodad
 ip -netns "${NS2}" addr add       fd::2/64 dev "${DEV}" nodad
 
-do_test() {
+run_test() {
 	local readonly IP="$1"
 	local readonly CLOCK="$2"
 	local readonly TXARGS="$3"
@@ -64,26 +65,46 @@ do_test() {
 	fi
 
 	local readonly START="$(date +%s%N --date="+ 0.1 seconds")"
+
 	ip netns exec "${NS2}" "${BIN}" -"${IP}" -c "${CLOCK}" -t "${START}" -S "${SADDR}" -D "${DADDR}" "${RXARGS}" -r &
 	ip netns exec "${NS1}" "${BIN}" -"${IP}" -c "${CLOCK}" -t "${START}" -S "${SADDR}" -D "${DADDR}" "${TXARGS}"
 	wait "$!"
 }
 
+do_test() {
+	run_test $@
+	[ $? -ne 0 ] && ret=1
+}
+
+do_fail_test() {
+	run_test $@
+	[ $? -eq 0 ] && ret=1
+}
+
 ip netns exec "${NS1}" tc qdisc add dev "${DEV}" root fq
+set +e
+ret=0
 do_test 4 mono a,-1 a,-1
 do_test 6 mono a,0 a,0
 do_test 6 mono a,10 a,10
 do_test 4 mono a,10,b,20 a,10,b,20
 do_test 6 mono a,20,b,10 b,20,a,20
 
 if ip netns exec "${NS1}" tc qdisc replace dev "${DEV}" root etf clockid CLOCK_TAI delta 400000; then
-	! do_test 4 tai a,-1 a,-1
-	! do_test 6 tai a,0 a,0
+	do_fail_test 4 tai a,-1 a,-1
+	do_fail_test 6 tai a,0 a,0
 	do_test 6 tai a,10 a,10
 	do_test 4 tai a,10,b,20 a,10,b,20
 	do_test 6 tai a,20,b,10 b,10,a,20
 else
 	echo "tc ($(tc -V)) does not support qdisc etf. skipping"
+	[ $ret -eq 0 ] && ret=$ksft_skip
 fi
 
-echo OK. All tests passed
+if [ $ret -eq 0 ]; then
+	echo OK. All tests passed
+elif [[ $ret -ne $ksft_skip && -n "$KSFT_MACHINE_SLOW" ]]; then
+	echo "Ignoring errors due to slow environment" 1>&2
+	ret=0
+fi
+exit $ret