Endpoint specific fixtures cannot be called by generic fixtures

[tanx16]

fuzz-lightyear has a test case,test_nested_endpoint_dependency_uses_default , which currently fails. We want to support this case in the future, but making this change is somewhat difficult. Here's the problem:

Consider the following:

• A nested_dependency that is registered for generic endpoints, returning caller * 2
• A caller that is registered for generic endpoints and new_opid, returning dependency + 1 and dependency + 2 respectively
• A dependency registered for generic endpoints that returns 1.

Now, when we call nested_dependency with the new_opid endpoint, we get 4 instead of 6. This is because nested_dependency is generic, so when we call register_factory it doesn't call inject_user_defined_variables with the list of operation_ids, since there is no list. Fixing this issue will require us to pass in the endpoint called into the wrapped function, and selecting which caller to use at that time.