Optimizer tests

[gAldeia]

This pull request adds new tests for optimization:

• Optimizing the weight w of a 3-ary Prod operator in two different settings: Prod(w*x1, x2, x3) and w*Prod(x1, x2, x3). To implement this test, I used more functionalities of the json parser (creating the node using its hash and arguments information, instead of letting it automatically infer it), which is working fine.
• A notable product data set x1^2 + 2*x1*x2 + x2^2 with a test designed for it;
• Optimizing the weight of a constant node.

These tests were added for both C++ and Python versions.

Note: the Prod tests uses the C++23 literal suffix for std::size_t when declaring the hash values (UZ suffixes at the end of hashes). For example: https://github.com/cavalab/brush/blob/2f1155100477289612337f6b657dfd9c52d5911c/tests/cpp/test_optimization.cpp?plain=1#L345-L359 This can be avoided if needed, just let me know.

Python tests were using fixtures and parameterized tests to avoid repeating the same structure over and over again. This pull request implements a fixture class and parameterized tests using Gtest.

After this, I plan to delete the optimizer_tests branch.

[lacava]

Thanks! I think we should avoid C++23, we're already having trouble supporting parts of C++20.

[gAldeia]

You are right. I removed the suffix feature from C++23 that I was using in the code!