fix(tests): installed_required_containers must not fail under set -e

The prior fix's loop `container_installed "$c" && echo "$c"` makes the
function's own exit status the exit status of its LAST array entry. If
that entry isn't installed on this node (e.g. required-stack-destructive's
array ends with mempool-api, absent on .5), the whole function reports
failure even though earlier entries matched fine — and under bats' set -e,
`targets="$(installed_required_containers)"` then aborts the test outright.
required-stack.bats got lucky (its array happens to end with an installed
container) but has the identical latent bug. Caught live on .5's iteration
3 of the multinode-pass gate run. Add explicit `return 0`.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/lifecycle/bats/required-stack.bats

@@ -56,6 +56,8 @@ installed_required_containers() {
   for c in "${required_containers[@]}"; do
     container_installed "$c" && echo "$c"
   done
+  # Always succeed — see the identical comment in required-stack-destructive.bats.
+  return 0
 }
 
 bitcoin_rpc() {