selftests/bpf: Add mptcp test with sockmap

Add test cases to verify that when MPTCP falls back to plain TCP sockets,
they can properly work with sockmap.

Additionally, add test cases to ensure that sockmap correctly rejects
MPTCP sockets as expected.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251111060307.194196-4-jiayuan.chen@linux.dev