Move tests into package.

[ceball]

No description provided.

[coveralls]

Coverage increased (+10.5%) to 78.244% when pulling 58cfaf3 on tests_into_pkg into eb0474e on master.

[ceball]

Comments on various pytest issues/PRs indicate that pytest does not support tests in package. Although in practice it seems to as we use that elsewhere. But no idea how to get it to work for testing plugins.