test: fix index.go to pass with recent index checks

test: fix index.go to pass with recent index checks

The compiler now gives an error for out of bounds constant
indexes for arrays, and for negative constant indexes for both
arrays and slices.

With this change the index.go test passes if CLs 6815085,
6815088, and 6812089 are committed.

R=golang-dev, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/6810085