Refactor tests to not test functions that ought to be private (but which are currently marked as public)

[yousefmoazzam]

The fix in this commit is a good example of an issue not being caught due to testing private functions (in particular, testing the parsing functions for lower-level AST node generation), but not testing the top-level parsing function: ed38cc2.

If the tests for the C AST node parsing were only testing the top-level function in the parse::c module, then that bug would have been caught, but instead it was missed because:

• the tests of the parsing function for statement AST nodes didn't require the distinction between a factor and an expression in order to pass
• but when running the top-level parsing function (via the binary crate) on a test C program using a single binary operation, that failed