Vincent,
It should be <include src="test"> instead of <include src="test.adp">.
Perhaps you should take a look at Using Templates in OpenACS 4.5 in the Developer's Guide. There's also The templates developer guide (I couldn't find it anywhere on openacs.org, so I linked to my own page).
Although all code should always be under version control when doing development work, I don't think you really need it if you're just making minor modifications. But I guess "creating new user pages" doesn't fall under that category. Perhaps look at CVS and 4.0 Client Development. Apart from what's outlined in the documentation, I don't know of any common way of conducting these matters.
Although you're not porting, Day in the Life of a Porter should give you some idea of the practices around here. And Lars also outlines his setup practices when doing client work in this article
