This refactoring of the tests of go/loader and refactor/rename made it
possible to write some loader tests I wanted, but the new tests reveal
bugs so they're commented out for now. I will fix them in a follow-up.
Change-Id: Iae3a20681a0a0791076debd4b82bb5ed74b0c577
Reviewed-on: https://go-review.googlesource.com/2825
Reviewed-by: Robert Griesemer <gri@golang.org>