[AttributeBundle] Write specs for the LoadMetadataSubscriber
[VariationBundle] Write specs for the LoadMetadataSubscriber
[AttributeBundle] Write specs for the container extension
[VariationBundle] Write common spec example for the container extension
Merge pull request #2226 from adamelso/spec-coverage-on-metadata-manipulation

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