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_fit.py b/test/py/tests/test_fit.py
index 459be5a..0f2d094 100755
--- a/test/py/tests/test_fit.py
+++ b/test/py/tests/test_fit.py
@@ -6,7 +6,7 @@
 import os
 import pytest
 import struct
-import u_boot_utils as util
+import utils as util
 import fit_util
 
 # Define a base ITS which we can adjust using % and a dictionary