tests: drop the external prefix in gu_msg()
tests: drop the external prefix in gu_msg()

This function is static so drop the prefix.

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

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