WP_UnitTestCase missing?

Question

I was looking at tests for wordpress and some, if not all the classes have WP_UnitTestCase but I cannot find the class, why do I care? I have downloaded The most up to date version of this from github which is the same set of tests.

Why does this matter so much? Because when ever I run one of the tests via phpunit filename.php I get the error from php unit stating that its missing WP_UnitTestCase.

So my question is:

Where is this class, why is it missing if it works for the WordPress developers?