ert: Where to define tests?

[Moritz Ulrich]

This question came up in irc today:

Where should one define ert tests?

- In a separate file, to keep the source clean?
- In the source files itself?
  - Should the file (require 'ert)?
  - (eval-after-load 'ert '(...))?

It doesn't look like this is documented. If there's a consent on this,
it should be documented in the ert Info page as well as in the info
page about packaging.

Cheers,
Moritz Ulrich