Hello,
When you say "stock version" are you talking about compiled binaries or building from the source code?
Working from binaries vs source code are two different things.
If you are working off of source code, the test folder would be tied to automated testing, such as Unit Testing, Functional Testing, and Integration Testing as described here:
https://www.labkey.org/Documentation/wiki-page.view?name=runningTests
The doc above goes over Functional Tests using the Selenium test framework.
Please note that Selenium doesn't support the latest version of Firefox at the moment, so if you are planning on doing Selenium testing on Firefox, you will need to use the Firefox ESR 45 version to test against.
Regards,
Jon