CTestDouble: Fix "surprising behavior" of --gen-pre-incl and post

[joakim-brannstrom]

The generation of the files need to be split from the above flags.

The intention of the flags --gen-pre-incl/post is to allow the user to inject "hooks" before and after the headers of what has been test doubled away.

Problem:

• The flags generate ready-to-use hooks with the content of the most common use case.
• In contrast to what dextool do in other cases the files are not overwritten in this case if they already exist.

Proposed solution:

• Introduce a new flag, --gen-cpp-hooks. It generates the default hooks that suppressed __cplusplus.
• Change --gen-pre-incl and post to not generate any files. To only inject the "#include"-hooks