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I'd propose (and have generally used, back when I worked on C projects) a fourth option that I don't think gets too far out of line.  If I understand the situation correctly, that you want to test how an "object" reacts under a barrage of test conditions, independent of the rest of the software, then you would probably have a better time writing a dedicated "test harness" for that object, stubbing out everything that it depends on and driving it with the code that you'd otherwise represent in option #3's header file.

This avoids the problem with the "include everything as a header" approach that violates the spirit of how things work, even if compilers find it permissible.  It doesn't risk shipping code accidentally, as you might with conditional directives, because those harnesses should live external to the main project and don't interact with the rest of the code.  And it gives you more control and a more complete example than just a header file.