selftests: mptcp: join: avoid backquotes

As explained on ShellCheck's wiki [1], it is recommended to avoid
backquotes `...` in favour of parenthesis $(...):

> Backtick command substitution `...` is legacy syntax with several
> issues.
>
> - It has a series of undefined behaviors related to quoting in POSIX.
> - It imposes a custom escaping mode with surprising results.
> - It's exceptionally hard to nest.
>
> $(...) command substitution has none of these problems, and is
> therefore strongly encouraged.

[1] https://www.shellcheck.net/wiki/SC2006



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>