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_source.py

@@ -3,7 +3,7 @@
 
 import os
 import pytest
-import u_boot_utils as util
+import utils as util
 
 @pytest.mark.boardspec('sandbox')
 @pytest.mark.buildconfigspec('cmd_echo')