Fixes many existing unit tests and enables PR unit test workflow

[kevinfjiang]

Tests not fixed:

OMOP and eICU datasets inherit from a deprecated base class that has had a majority of its functionality stripped away. Those datasets are non-functional and unittests will be reenabled at a later date
test_mortality_prediction relies on datasets that are require additional credentials to view. These unit tests are skipped for now until a solution on how to solve the credentials issue is implemented

Closes: #425
Closes: #338

The github actions script ran correctly according to the following action link which ran when I created a pull request from the branch feat/pr_test to master

This obsoletes #514 since I had to make some manually resolves merge conflicts

[jhnwu3]

Hey Kevin, thanks for the effort, just a heads-up, we've moved towards all of the usual tests into tests/core. Paul helped us setup an automated CI. Let us know if this helps redirect some of the efforts since we definitely would like to have a more comprehensive usable set of test cases.

Oh btw, join us on Discord if you'd like to chat more. It's easier for us and we're more accessible as the pull requests section of GitHub is kind of a mess atm.