Exclude tests package from built package

[aldenrogers]

Describe the bug
Currently setup.py will find and package everything, including the tests package. This is bad because it clutters the top-level namespace and can cause name conflicts if multiple packages include an identical name.

To Reproduce
To see this, build a wheel with

$ python -m pip install build
$ python -m build --wheel

Then list the contents of the wheel with your favorite zip program, e.g.

$ 7z l dist/causalml-0.12.3-cp310-cp310-linux_x86_64.whl
... (more files)
2022-04-25 19:18:56 .....         4210         1059  causalml/optimize/utils.py
2022-04-25 19:18:56 .....         4079         1255  causalml/optimize/value_optimization.py
2022-04-25 19:18:56 .....            0            2  tests/__init__.py
2022-04-25 19:18:56 .....         1432          365  tests/conftest.py
2022-04-25 19:18:56 .....          292          189  tests/const.py
2022-04-25 19:18:56 .....         1412          592  tests/test_cevae.py
... (etc.)

Expected behavior
Built wheel only contains dist-info files and causalml package.

Environment (please complete the following information):

• OS: Arch linux
• Python Version: 3.10
• Versions of Major Dependencies (pandas, scikit-learn, cython): pandas==1.4.2, scikit-learn==1.0.2, cython==0.29.28

Additional context
I'll submit a PR with a fix momentarily.