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_hashes.py b/test/py/tests/test_fit_hashes.py
index 7bc24a7..0b3c85f 100644
--- a/test/py/tests/test_fit_hashes.py
+++ b/test/py/tests/test_fit_hashes.py
@@ -12,7 +12,7 @@
 
 import os
 import pytest
-import u_boot_utils as util
+import utils as util
 
 kernel_hashes = {
     "sha512" : "f18c1486a2c29f56360301576cdfce4dfd8e8e932d0ed8e239a1f314b8ae1d77b2a58cd7fe32e4075e69448e623ce53b0b6aa6ce5626d2c189a5beae29a68d93",