improve assertion generation by asserting on the receiver object

improve assertion generation by asserting on the receiver object.

* this would make a potentially stronger assertion on all tests
* this would enable to generate tests for void methods

we could also assert the state of the parameters after the call

The issue names no file, test, or entry point. Start by locating the assertion-generation implementation and its tests, then clarify whether completion requires receiver-state assertions for all generated tests, support for void methods, and assertions on parameter state after calls.

Written by the indexing model from the issue text.