selftests: mptcp: declare var as local

Just to avoid classical Bash pitfall where variables are accidentally
overridden by other functions because the proper scope has not been
defined.

That's also what is done in other MPTCP selftests scripts where all non
local variables are defined at the beginning of the script and the
others are defined with the "local" keyword.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>