Closes webonyx#728

This ports the JavaScript tests for `RFC: Allow interfaces to
implement other interfaces` to PHP. This should ensure that there is
sufficient test coverage for the changes made to support interfaces
implementing interfaces.

Tests taken from https://github.com/graphql/graphql-js/pull/2084/files
including any typoes in test description strings to aid in comparison.

This is part of the update to allow interfaces to implement interfaces.
A single extend statement to add an implementation of an interface
without field declarations is valid. This was caught by tests and brings
in a change from graphql/graphql-js#2084

Part of the work done to implement interfaces implementing interfaces.
This was caught by test and improves on the previously done changes for
the SchemaValidationContext by implementing
`validateTypeImplementsAncestors` which was missing.

This redoes the work done for the Schema class since it was previously
guessed at. It now more closely follows graphql/graphql-js/pull/2084

Co-authored-by: Benedikt Franke <[email protected]>

# Conflicts:
#	src/Utils/SchemaPrinter.php

Removing a public constant is a breaking change and can not be
implemented in a minor version. Instead the internal value is changed to
ensure that existing code keeps working with the new interface
implementation logic.