selftests: mptcp: join: uniform listener tests

The alignment was different from the other tests because tabs were used
instead of spaces.

While at it, also use 'echo' instead of 'printf' to print the result to
keep the same style as done in the other sub-tests. And, even if it
should be better with, also remove 'stdbuf' and sed's '--unbuffered'
option because they are not used in the other subtests and they are not
available when using a minimal environment with busybox.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 178d0232

 ("selftests: mptcp: listener test for in-kernel PM")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>