Improves test coverage of the rules added in CL 63795 and would have
detected the bug fixed by CL 63950.

Change-Id: I107ee8d8e0b6684ce85b2446bd5018c5a03d608a
Reviewed-on: https://go-review.googlesource.com/64130Reviewed-by: Keith Randall <khr@golang.org>