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

test/py/tests/test_android/test_avb.py

@@ -15,7 +15,7 @@ For configuration verification:
 """
 
 import pytest
-import u_boot_utils as util
+import utils as util
 
 # defauld mmc id
 mmc_dev = 1