[CN-Exec] Preprocessor directives for better testing support

[ZippeyKeys12]

Right now there is a --with-test-gen flag, which throws an error if there is a main function.

Instead, there should be a more generally named --with-testing (or something more descriptive) flag which performs at least the following:

• Allows the main function to be absent
• If a main function is present, wraps it in #ifndef CN_TESTING_MODE
• Based on CN_TESTING_MODE, for a given static global static <ty> <ident>;, has extern <ty> <ident>; in cn.h and <ty> <ident>; in cn.c instead.
• For static functions, wrap these modifiers in #ifndef CN_TESTING_MODE

Since these changes would be behind a compiler flag, they could in theory be enabled by default rather than by a flag. However, this could add unnecessary noise to the instrumented files.

The end result is that compiling the instrumented file with CN_TESTING_MODE will allow various forms of testing to link against functions and globals where they otherwise could not.

CN-Test-Gen currently works around this by just acting as an addendum to the instrumented file, which is far from ideal.

Once implemented, CN-Test-Gen (and other testing efforts) can be updated to work with the new flag and --with-test-gen can eventually be removed.

[dc-mak]

Can also use line directives to not mess up source location info too much.