The variadic handle::operator() offers the same functionality as well
as mixed positional, keyword, * and ** arguments. The tests are also
superseded by the ones in `test_callbacks`.
The C++ part of the test code is modified to achieve this. As a result,
this kind of test:
```python
with capture:
kw_func1(5, y=10)
assert capture == "kw_func(x=5, y=10)"
```
can be replaced with a simple:
`assert kw_func1(5, y=10) == "x=5, y=10"`
Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.
This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.
