Defaulting tests internal to the package

[Nate Finch]

[reposting this to the wider juju-dev mailing list]

So, I hit an interesting problem a while back.  I have some unit tests that
need to be internal tests, thus they are in 'package foo'.  However, my
code needs to use some testhelper functions that someone else wrote...
which are in 'package foo_test'.  It's impossible to reference those, so I
had to move those helpers to package foo, which then either requires that
we make them exported (which is exactly like using export_test.go, which,
in the juju-core Oakland sprint, we all agreed was bad), or all tests that
use the helpers need to be in 'package foo'... which means I had to go
change a bunch of files to be in package foo, and change the calls in those
files from foo.SomeFunc() to just SomeFunc().

Given the assumption that some tests at some point will make sense to be
internal tests, and given it's likely that helper functions/types will want
to be shared across suites - should we not just always make our tests in
package foo, and avoid this whole mess in the first place?

(A note, this happened again today - I wanted to add a unit test of a
non-exported function to an existing test suite, and can't because the unit
tests are in the foo_test package)

There seems to only be two concrete benefits to putting tests in package
foo_test:
1. It avoids circular dependencies if your test needs to import something
that imports the package you're testing.
2. It makes your tests read like normal usages of your package, i.e.
calling foo.SomeFunc().
The first is obviously non-negotiable when it comes up... but I think it's
actually quite rare (and might indicate a mixture of concerns that warrants
investigation).  The second is nice to have, but not really essential (if
we want tests that are good examples, we can write example functions).

So, I propose we put all tests in package foo by default.  For those devs
that want to only test the exported API of a package, you can still do
that.  But this prevents problems where helper code can't be shared between
the two packages without ugliness and/or dumb code churnm, and it means
anyone can add unit tests for non-exported code without having to create a
whole new file and testsuite.

-Nate
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