selftests: mptcp: join: create tmp files only if needed

These tmp files will only be created when a test will be launched.

This avoid 'dd' output when '-h' is used for example.

While at it, also avoid creating netns that will be removed when
starting the first test.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>