test: fix return.go to not use fallthrough in a type switch

The gc compiler only gives an error about fallthrough in a
type switch if it has not given any errors in an earlier pass.
Remove all functions in this test that use fallthrough in a
type switch because they don't test anything useful and they
cause gccgo to give unexpected errors.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12614043