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.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -2177,8 +2177,8 @@ 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
+		FAILING_LINKS="1" \
 			run_tests $ns1 $ns2 10.0.1.1 1
 		chk_join_nr 2 2 2
 		chk_add_nr 1 1
@@ -2193,7 +2193,7 @@ 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"
+		FAILING_LINKS="1 2" \
 			run_tests $ns1 $ns2 10.0.1.1 1
 		chk_join_nr 2 2 2
 		chk_add_nr 1 1
@@ -2209,7 +2209,7 @@ 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"
+		FAILING_LINKS="1 2" \
 			run_tests $ns1 $ns2 10.0.1.1 2
 		chk_join_nr 2 2 2
 		chk_add_nr 1 1