The product test library, at account.development.product.test, contains tests for your product. In particular, it should contain regression tests for custom scripts.
See Test for an example test folder.
The sample shows an effective way of settings out the tests. Split the test into different suites, one for each script or set of related scripts. In the sample, there is one suite, the Gadgets summary test.
Inside this, have a folder for the data. Use the Stale Folder Listing type so that changes to the data do not propagate out of the test data. Also have a folder for the test runs, of type Test Package.
Split the suite into a series of test runs. Each test run should be represented by one node or folder in the data folder, and one test of type Test in the tests folder. Use the same name and local reference for the data node/folder and the test.
See Test library for a brief guide to the regression test harness.
Within the individual tests, locate the data node/folder by binding to the overall data folder, and then using a literal to find the node/folder within this. This ensures that the test is not invalidated (and marked as requiring a rerun) when the test data changes.