Enable assertCallsFunction to count the number of times that the target function was called

[Duckwhale]

One of the rarely-used assertions is assertCallsFunction, which has a niche use case in test cases, mostly for verifying side effects. Currently, it uses debug hooks to check that the target function is called (once). Ideally, it should check the count (added parameter) and allow the user to optionally assert that the expected number of calls was made - defaulting to one.