Expand testing

Currently the test suite is very small, most notably the juliacall suite: https://github.com/JuliaPy/PythonCall.jl/blob/main/pytest/test_all.py which is only 19 lines long. I am very worried about reliability issues from this and whether this might result in regressions.

I think expanding the test suite should be a priority, especially on the juliacall side, before new features are worked on.

Here are some suggestions. Feel free to check any that are already done or add more

- [ ] Unittests for features shown in the docs
- [ ] ALL JuliaCall guide examples (https://juliapy.github.io/PythonCall.jl/stable/juliacall/)
- [ ] ALL PythonCall guide examples (https://juliapy.github.io/PythonCall.jl/stable/pythoncall/)
- [ ] ALL conversion rules Python -> Julia (https://juliapy.github.io/PythonCall.jl/stable/conversion-to-julia/)
- [ ] ALL conversion rules Julia -> Python (https://juliapy.github.io/PythonCall.jl/stable/conversion-to-python/)
- [ ] Miscellaneous unittests
- [ ] GC tests (currently there are zero)
- [ ] Dummy package with pyjuliapkg and `newmodule` used
- [ ] Multithreading unittests (with PythonCall.GC.disable())
- Need to launch additional test suite in CI, with signals turned on
- [ ] Distributed compute unittests
- [ ] Julia's stdlib tests from Python via juliacall (copy the test suite; disable everything, and gradually turn on tests)
- [ ] Python's stdlib tests from Julia via PythonCall.jl (copy the test suite; disable everything, and gradually turn on tests)
- [ ] Any areas missing due to coverage milestones:
- [ ] Code coverage milestones
- [ ] Find way to track Python code generated from Julia code
- [ ] Reach 60% code coverage on Python source code
- [ ] Reach 80% code coverage on Python source code
- [ ] Reach 90% code coverage on Python source code
- [ ] Reach 100% code coverage on Python source code
- [ ] Set up code coverage for Julia source code (currently not tracked)
- [ ] Reach 60% code coverage on Julia source code
- [ ] Reach 80% code coverage on Julia source code
- [ ] Reach 90% code coverage on Julia source code
- [ ] Reach 100% code coverage on Julia source code
- [ ] Add integration tests
- [ ] Numpy
- [ ] PythonPlot.jl
- [ ] Pandas (https://juliapy.github.io/PythonCall.jl/stable/compat/#Tabular-data-/-Pandas)
- [ ] SymPy.jl (whenever PythonCall.jl version is made)
- [ ] Matplotlib interactive support (harder to test, but you would want to test the features explained here – https://juliapy.github.io/PythonCall.jl/stable/compat/#MatPlotLib-/-PyPlot-/-Seaborn)
- [ ] IPython support
- [ ] PySR
- [ ] diffeqpy
- [ ] PyTorch import issue (basically just verify the solution on https://juliapy.github.io/PythonCall.jl/stable/faq/)