src: use internal tests if possible

If a test can be placed in the same package ("internal"), it is placed
there. This facilitates testing of package-private details. Because of
dependency cycles some packages cannot be tested by internal tests.

R=golang-dev, rsc, mikioh.mikioh
CC=golang-dev, r
https://codereview.appspot.com/7323044

Committer: Russ Cox <rsc@golang.org>