Support for top-down testing

[racoltdev]

Current implementation supports bottom-up testing strategies, but a top-down strategy is difficult or impossible without stubs.

Possible methods of implementation:

• regenerate .c/pp files with stubs
• interface directly with object code

Regenerate src files seems more stable, safer, and easier, but is likely less performant. More research should be done.

[racoltdev]

Inter-file stubs can be implemented by modifying what imports link to.
This can be done by

• editing source code import statements
• creating new implementation files and moving/renaming the old ones
• creating new implementation files and interfacing with the linker during compilation to link mock implementations

Same file stubs would require one of two options listed in opening comment.