selftests: mptcp: join: mark 'fastclose' tests as flaky

These tests are flaky since their introduction. This might be less or
not visible depending on the CI running the tests, especially if it is
also busy doing other tasks in parallel, and if a debug kernel config is
being used.

It looks like this issue is often present with the NetDev CI. While this
is being investigated, the tests are marked as flaky not to create
noises on such CIs.

Fixes: 01542c9b ("selftests: mptcp: add fastclose testcase")
Link: https://github.com/multipath-tcp/mptcp_net-next/issues/324Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240524-upstream-net-20240524-selftests-mptcp-flaky-v1-3-a352362f3f8e@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -261,6 +261,8 @@ reset()
 
 	TEST_NAME="${1}"
 
+	MPTCP_LIB_SUBTEST_FLAKY=0 # reset if modified
+
 	if skip_test; then
 		MPTCP_LIB_TEST_COUNTER=$((MPTCP_LIB_TEST_COUNTER+1))
 		last_test_ignored=1
@@ -448,7 +450,9 @@ reset_with_tcp_filter()
 # $1: err msg
 fail_test()
 {
-	ret=${KSFT_FAIL}
+	if ! mptcp_lib_subtest_is_flaky; then
+		ret=${KSFT_FAIL}
+	fi
 
 	if [ ${#} -gt 0 ]; then
 		print_fail "${@}"
@@ -3069,6 +3073,7 @@ fullmesh_tests()
 fastclose_tests()
 {
 	if reset_check_counter "fastclose test" "MPTcpExtMPFastcloseTx"; then
+		MPTCP_LIB_SUBTEST_FLAKY=1
 		test_linkfail=1024 fastclose=client \
 			run_tests $ns1 $ns2 10.0.1.1
 		chk_join_nr 0 0 0
@@ -3077,6 +3082,7 @@ fastclose_tests()
 	fi
 
 	if reset_check_counter "fastclose server test" "MPTcpExtMPFastcloseRx"; then
+		MPTCP_LIB_SUBTEST_FLAKY=1
 		test_linkfail=1024 fastclose=server \
 			run_tests $ns1 $ns2 10.0.1.1
 		chk_join_nr 0 0 0 0 0 0 1