Gain access to input variables from mock functions

**Is your feature request related to a problem? Please describe.**

During testing, I need to make sure certain requests are being called with specific variables, similar to how you'd assert functions were called with certain arguments.

**Describe the solution you'd like**
Pass the input variables of the operation to the mocked function, only if it's a mock for an operation (mutation/query), not just any field.

**Describe alternatives you've considered**
I tried using resolvers, which seem to be getting the input variables through the info parameter, but resolvers have to supply all the fields for a given operation, whereas I want to supply only partial fields and let the rest of the fields be populated using the current mocking strategy.

Start by tracing the mock-function handling for GraphQL operations, focusing on how query and mutation mocks receive inputs. Verify that operation variables reach those mocks while field mocks retain the existing partial-field behavior, and add or update coverage for asserting variables passed to a mocked operation.

Written by the indexing model from the issue text.