Allow custom apps to opt into app tests

[zyf0717]

Summary

• Allow test.json files to explicitly opt custom apps into bin/runapptests.js by setting allowCustomApp.
• Keep the existing default behavior for custom apps that do not opt in.
• Fix the custom-app error path to report app.id instead of the undefined appId variable.

Rationale

Some custom apps still have deterministic packaged defaults and emulator-testable storage/module behavior. This keeps custom apps blocked by default while allowing app authors to opt into the documented test runner when their test intentionally covers the metadata/default-storage path rather than customizer-generated content.

Validation

• node -c bin/runapptests.js
• node bin/runapptests.js --id demoappfortestformat
• git diff --check

[bobrippling]

Sounds good, tagging @gfwilliams with this being a more core change

[gfwilliams]

Looks good - thanks! Does this need documenting somewhere though?