selftests: mptcp: join: make it shellcheck compliant

This fixes a few issues reported by ShellCheck:

- SC2068: Double quote array expansions to avoid re-splitting elements.
- SC2206: Quote to prevent word splitting/globbing, or split robustly
          with mapfile or read -a.
- SC2166: Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.
- SC2155: Declare and assign separately to avoid masking return values.
- SC2162: read without -r will mangle backslashes.
- SC2219: Instead of 'let expr', prefer (( expr )) .
- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly
          with $?.
- SC2236: Use -n instead of ! -z.
- SC2004: $/${} is unnecessary on arithmetic variables.
- SC2012: Use find instead of ls to better handle non-alphanumeric
          filenames.
- SC2002: Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..'
          instead.

SC2086 (Double quotes to prevent globbing and word splitting) is ignored
because it is controlled for the moment and there are too many to
change.

While at it, also fixed the alignment in one comment.
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>