Don't force users to match existing config

If an object is in the registry already, and no bespoke config has been
specified - don't consider that an exceptional case.
add tests demonstrating how helper loading works

This applies to all object registries
better assert logic

the asserts aren't strictly necessary, but since the test is checking
that no exception is thrown, do something a little more logical
phpcs fix
Merge pull request #5288 from AD7six/3.0-leniant-duplicate-check

Don't force users to match existing config