selftests: mptcp: join: fix ShellCheck warnings

Most of the code had an issue according to ShellCheck.

That's mainly due to the fact it incorrectly believes most of the code
was unreachable because it's invoked by variable name, see how the
"tests" array is used.

Once SC2317 has been ignored, three small warnings were still visible:

 - SC2155: Declare and assign separately to avoid masking return values.

 - SC2046: Quote this to prevent word splitting: can be ignored because
   "ip netns pids" can display more than one pid.

 - SC2166: Prefer [ p ] || [ q ] as [ p -o q ] is not well defined.

This probably didn't fix any actual issues but it might help spotting
new interesting warnings reported by ShellCheck as just before,
ShellCheck was reporting issues for most lines making it a bit useless.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>