previews exported files, adds preview tests

[Fil]

closes #1694

[Fil]

It's not something that we're doing for parameterized pages and resources, i.e. you can call page-foo if there is a page-[var].md, even if that parameter is not listed in dynamicPaths.

Maybe we should (at least) log a line of warning? Another approach might be to return a 404 with an explicit message that says “this resource exists and we could serve it if you declared it in dynamicPaths”… or even maybe a button to bypass that problem, when you're developing something and don't want to get thrown out of your flow.