Add CI job to check all test files are included in package

[freddyaboulton]

We saw in #1024 that we were not adding test files to the pypi package. This makes it impossible to test evalml during conda package creation and it also doesn't let users test the package once they pip install it (when tests are included in the package, users can test it with pytest --pyargs evalml).

Going forward, we want to make sure that all current and future test files are included in the package. This issues tracks works to create a CI Job to check that every test file is included in the package.

I think checking that every directory with the name test_ has an init file would suffice but that is not the only solution.

[dsherry]

Agreed @freddyaboulton , let's add CI coverage for this.

I like your idea of looking for every directory with name test_ under evalml/tests/ and making sure each has a file called __init__.py. I think that would do it.