Include flag for test dependencies

[Phidelux]

I am currently building a project that has a development dependency on mockall, which should not be included bitbake recipe. Removing the mockall crates from the list of source crates in the resulting bitbake recipe is not that of a big issue, but mockall itself pulls in additional dependencies. This makes cleaning up the resulting recipe a very tedious process. It would be nice if the development dependencies are excluded from the recipe by default and could be included with a new flag. If this is too restrictive a flag excluding the development dependencies could also do the trick.

Thanks.