tests: new assertion helper
tests: new assertion helper

Add the GU_ASSERT_RET_OK() macro which checks if an integer (normally
returned from a function call) equals 0 - indicating a correct result.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod