fix: ensure we redirect stderr to stdout for test

tests/unit/40_core_2.bats

@@ -41,7 +41,7 @@ build_nginx_config() {
   echo "output: "$output
   echo "status: "$status
   assert_failure
-  run /bin/bash -c "dokku fakecommand | grep -q 'is not a dokku command'"
+  run /bin/bash -c "dokku fakecommand 2>&1 | grep -q 'is not a dokku command'"
   echo "output: "$output
   echo "status: "$status
   assert_success