consensus on using separate *_test packages

[Eric Snow]

In Go we put tests in *_test.go files that are only built when
testing.  There are two conventions for what package to declare in
those files (relative to the corresponding non-test package):
"<package>" and "<package>_test".  In our code we have a mix of both.
This can be confusing.  We should come to a consensus on which to use
and stick with it, both for new code and when given the opportunity
while refactoring.  (As usual, a whole-sale refactor wouldn't be a
good idea).

This came up on IRC a couple months back. [1]  At the time I referred
to a post from Gustavo on golang-nuts where he stated a preference for
using a separate package for *_test.go files. [2]  Also note that
there are some cases where test files must be in a separate package
(e.g. to break circular imports).  So unless we always use separate
test packages, we will almost certainly end up with a mix (which is
exactly the issue at hand!).

>From my perspective, I agree with Gustavo that using a separate _test
package is a good idea.  So I would favor us being consistent in doing
that. :)

-eric

[1] http://irclogs.ubuntu.com/2014/07/22/%23juju-dev.html#t15:36
[2] https://groups.google.com/forum/#!msg/Golang-nuts/dkk0X1tIs6k/nO3CKFqbIxYJ