Racket: new commands to show and hide test submodules

The new commands, being racket-specific, are called
geiser-racket-{show,hide,toggle}-tests, and have no default binding in
geiser-mode (since they don't have any meaning in Guile).

The implementation is based on more generic functions in geiser-edit
that allow hiding of any top-level form, given its name, so we will
probably find new forms to hide in the future.

Hiding is limited to top-level forms, which i think is fine for the only
use case we have in mind right now.