selftests: mptcp: set FAILING_LINKS in run_tests

Set FAILING_LINKS as an env var with a limited scope only when calling
run_tests().

Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230623-send-net-next-20230623-v1-3-a883213c8ba9@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: geliangtang

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -2177,9 +2177,9 @@ link_failure_tests()
 		pm_nl_set_limits $ns1 0 2
 		pm_nl_add_endpoint $ns1 10.0.2.1 dev ns1eth2 flags signal
 		pm_nl_set_limits $ns2 1 2
-		FAILING_LINKS="1"
 		pm_nl_add_endpoint $ns2 10.0.3.2 dev ns2eth3 flags subflow,backup
-		run_tests $ns1 $ns2 10.0.1.1 1
+		FAILING_LINKS="1" \
+			run_tests $ns1 $ns2 10.0.1.1 1
 		chk_join_nr 2 2 2
 		chk_add_nr 1 1
 		chk_link_usage $ns2 ns2eth3 $cinsent 0
@@ -2193,8 +2193,8 @@ link_failure_tests()
 		pm_nl_add_endpoint $ns1 10.0.2.1 dev ns1eth2 flags signal
 		pm_nl_set_limits $ns2 1 2
 		pm_nl_add_endpoint $ns2 10.0.3.2 dev ns2eth3 flags subflow,backup
-		FAILING_LINKS="1 2"
-		run_tests $ns1 $ns2 10.0.1.1 1
+		FAILING_LINKS="1 2" \
+			run_tests $ns1 $ns2 10.0.1.1 1
 		chk_join_nr 2 2 2
 		chk_add_nr 1 1
 		chk_stale_nr $ns2 2 4 2
@@ -2209,8 +2209,8 @@ link_failure_tests()
 		pm_nl_add_endpoint $ns1 10.0.2.1 dev ns1eth2 flags signal
 		pm_nl_set_limits $ns2 1 3
 		pm_nl_add_endpoint $ns2 10.0.3.2 dev ns2eth3 flags subflow,backup
-		FAILING_LINKS="1 2"
-		run_tests $ns1 $ns2 10.0.1.1 2
+		FAILING_LINKS="1 2" \
+			run_tests $ns1 $ns2 10.0.1.1 2
 		chk_join_nr 2 2 2
 		chk_add_nr 1 1
 		chk_stale_nr $ns2 1 -1 2