Reduce the number of extensions in the system

[guillep]

Many extesions are:

• tests extending existing test classes that can be either integrated in the extended class or put in their own class
• extensions that could be integrated in the extended class but maybe were not because of historical reasons
• extensions that are used in a single place, have only that single use case, and can just be inlined in the caller

This PR solves ~300 extensions that look unjustified out of ~5K using the simple heuristics above:

• move to the extended class
• inline in caller
• put in their own class