util/u_atomic: Add a simple test.

util/u_atomic: Add a simple test.

It was much easier for me to verify things build and run as expected
with this simple test, than building and testing whole Mesa.

With scons the test can be build and run merely by doing:

  scons u_atomic_test

Building the test with autotools is left as a future exercise.

Reviewed-by: Matt Turner <mattst88@gmail.com>