macro: use unique variable names for math-macros

Similar to container_of(), we now use unique variable names for the bascic
math macros MAX, MIN, CLAMP, LESS_BY. Furthermore, unit tests are added to
verify they work as expected.

For a rationale, see:
    commit fb835651aff79a1e7fc5795086c9b26e59a8e6ca
    Author: David Herrmann <dh.herrmann@gmail.com>
    Date:   Fri Aug 22 14:41:37 2014 +0200

        shared: make container_of() use unique variable names