We have not yet settled on the meaning of Package.IsTest, and more
deeply which packages are test and which are non test packages.
This change will remove Package.IsTest in the meantime to avoid
confusion. We may later add a boolean or some other way to distinguish
test and non-test packages.
Change-Id: If6b128f7914009fdd42b8bc3de3bff73c8f006cd
Reviewed-on: https://go-review.googlesource.com/124388
Reviewed-by: Russ Cox <rsc@golang.org>