unit-tests: Use function pointers to test generic return_* helper functions

unit-tests: Use function pointers to test generic return_* helper functions

These functions are declared without arguments, passing arguments to them
causes warnings such as the following with newer compilers:

  passing arguments to 'return_null' without a prototype is deprecated in all versions of C and is not supported in C2x [-Werror,-Wdeprecated-non-prototype]

We only use them via function pointers, which doesn't trigger any warnings
and hopefully continues to work.