test/py: Drop u_boot_ prefix on test files

We know this is U-Boot so the prefix serves no purpose other than to
make things longer and harder to read. Drop it and rename the files.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # test_android / test_dfu
diff --git a/test/py/tests/test_gpio.py b/test/py/tests/test_gpio.py
index 5c9d0b6..46b674b 100644
--- a/test/py/tests/test_gpio.py
+++ b/test/py/tests/test_gpio.py
@@ -5,7 +5,7 @@
 
 import pytest
 import time
-import u_boot_utils
+import utils
 
 """
 	test_gpio_input is intended to test the fix 4dbc107f4683.