selftests: mptcp: join: check for tools only if needed

To allow showing the 'help' menu even if these tools are not available.

While at it, also avoid launching the command then checking $?. Instead,
the check is directly done in the 'if'.

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>