Refactor/move extension factories included in `lifecycle.test.ts`

[mnholtz]

There are two factories included in lifecycle.test.ts for generating a trigger extension point, and an extension: https://github.com/pixiebrix/pixiebrix-extension/blob/feature%2F5937_fix_mod_deployment_updates/src/contentScript/lifecycle.test.ts#L43-L77

The extension point factory id generator seems to be broken (all extension point ids for extension points generated consecutively seem to be test/extension-point-1 instead of test/extension-point-1, test/extension-point-2 ... test/extension-point-n); in addition, passing an id override seems to be ineffective.

• [ ] Fix the extension point/extension factories so that iteration/overrides work as expected
• [ ] and move these factories out of the lifecycle test into a dedicated factory file