tests: devicetree: Fix compile errors with GPIO define

A number of HALs define GPIO so we get a conflict in error of the form:

   main.c:12: error: "GPIO" redefined [-Werror]

Fix this by rename the defines in the test to "TEST_".  So GPIO becomes
TEST_GPIO.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>