[release-branch.go1.5] cmd/go: disable TestNoteReading on dragonfly

It started failing on the dragonfly builder at an unrelated commit
(one that changed the wording in a few comments in the compiler).

Created #13364 to track this.

Change-Id: I462880bed8ff565a9950e7e185de97d43999c5e2
Reviewed-on: https://go-review.googlesource.com/17143


Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/17144