selftests: mptcp: lib: fix shellcheck errors

selftests: mptcp: lib: fix shellcheck errors

It looks like we missed these two errors recently:

  - SC2068: Double quote array expansions to avoid re-splitting elements.
  - SC2145: Argument mixes string and array. Use * or separate argument.

Two simple fixes, it is not supposed to change the behaviour as the
variable names should not have any spaces in their names. Still, better
to fix them to easily spot new issues.

Fixes: f265d3119a29 ("selftests: mptcp: lib: use setup/cleanup_ns helpers")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240712-upstream-net-next-20240712-selftests-mptcp-fix-shellcheck-v1-1-1cb7180db40a@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>