docs specify file that does not exists

The testing docs [alt/docs/testing/stores.md](https://github.com/goatslacker/alt/blob/master/docs/testing/stores.md) use `alt/utils/AltTestingUtils` but I believe that file was removed a long time ago (in 0844b3aa2fc6de2034e8e01aa8c8b9e13839d755)
Is there another way to test stores?

Start with docs/testing/stores.md and inspect commit 0844b3aa2fc6de2034e8e01aa8c8b9e13839d755 to confirm when alt/utils/AltTestingUtils was removed. Identify the currently supported way to test stores, then update the documentation so it no longer references the missing file and clearly describes the replacement.

Written by the indexing model from the issue text.