Tests for environment variables utility functions

This is the first patch from the new set of tests for Bionic standard functions.
Patch also contains snippets.h with macro definitions for ongoing tests contribution from Intel.

Change-Id: Ie568788a24832394e597ad33f44a5c71cb33b51f
Signed-off-by: Grigoriy Kraynov <grigoriy.kraynov@intel.com>