selftests: mptcp: userspace: print titles

This script is running a few tests after having setup the environment.

Printing titles helps understand what is being tested.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>