Test code should be separate from module code

[thejcannon]

(Copied from #1473)

Having the tests be in the same file as the source code is not something I'd expect when installing a Python module.

• It pollutes the API of the module (attribute discovery will show a bunch of unrelated functions)
• It can pollute the module's dependencies
• It bloats the installed modules (more LOC to download and sift through)
• It makes features such as pytest's conftest.py harder to use.

If you want people to be able to run tests after installation you should allow installation using an extra (like nidmm[test])

[sbethur]

#1477 should fix this issue for converters. With that, none of the "shipping" modules should have this problem. But there are other modules in helper folder, which are used during codegen, that we need to fix.

[thejcannon]

:rocket: