test-listener-manager can fail if the kernel doesn't support MPTCP

[matttbe]

Describe the bug The Debian CI reported a errors when validating the unit tests for mptcpd 0.11: the test test-listener-manager fails with:

Unable to open MPTCP listener: Protocol not supported
test-listener-manager: test-listener-manager.c:106: test_listen: Assertion `mptcpd_lm_listen(_lm, sa) == 0' failed.

To Reproduce I guess it can be reproduced using a non MPTCP kernel

Expected behavior Like some other tests, the test should be skip if we notice the kernel doesn't support MPTCP.

Desktop (please complete the following information):

• Linux Kernel: I don't know :-/ at some points, they were using v4.19 kernels
• Compiler: gcc 12

[matttbe]

Thanks! :)