python api unit tests for Op (#6785)

python api unit tests for Op (#6785)

Unit tests are translated from https://github.com/cvc5/cvc5/blob/master/test/unit/api/op_black.cpp
to python.

The only thing that is not faithfully translated is that the `cpp` tests expect the template function `[getIndices](https://github.com/cvc5/cvc5/blob/master/src/api/cpp/cvc5.h#L841)` to fail when an inappropriate type is given to it, while in the [`python` API](https://github.com/cvc5/cvc5/blob/90d19f7cdbaf41e389bdcbd099471f658a35ce98/src/api/python/cvc5.pxi#L343) this function is not a template, but just tries every supported type.

For example, the following line is not translated:
https://github.com/cvc5/cvc5/blob/90d19f7cdbaf41e389bdcbd099471f658a35ce98/test/unit/api/op_black.cpp#L206