selftests: mptcp: add checks for fallback counters

selftests: mptcp: add checks for fallback counters

Recently, some mib counters about fallback has been added, this patch
provides a method to check the expected behavior of these mib counters
during the test execution.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/571
Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250902-net-next-mptcp-misc-feat-6-18-v2-2-fa02bb3188b1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>