[AttributeBundle][VariationBundle] Spec coverage on Doctrine metadata subscribers and container extensions

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

I could sort of see what these subscribers were doing, but it's not immediately clear.

By adding specs, I have hopefully described their behavior well.