Make rdt_stub compatible with code templates

[kalekundert]

The trick is that rdt_stub goes beyond just evaluating a single template. It also:

• Creates corresponding unit test files.
• Edits the corresponding *.src.settings and *.test.settings files.

It'd be nice to use the code templates, because they're more standard, and there are more of them. And editing the settings files isn't really a barrier, since that's not really dependent on the how the templates are done. But I need a way to link a code template with a test template.

My first thought is to embed an optional config file at the same level as the templates themselves. If there's a config file, it can specify a non-default test template (or whether there should even be a test template). By default, the standard unit test template will be used.

[kalekundert]

Ah, rdt_stub also edits registrator files, but only for movers. I'd also need to think of a general way to do that...