selftests: mptcp: add subflow limits test-cases

Add and delete a bunch of endpoints and verify the
respect of configured limits.

This covers the codepath introduced by the previous patch.

Fixes: 69c6ce7b

 ("selftests: mptcp: add implicit endpoint test case")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>