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_pinmux.py b/test/py/tests/test_pinmux.py
index f8c3f0d..ee79e84 100644
--- a/test/py/tests/test_pinmux.py
+++ b/test/py/tests/test_pinmux.py
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 
 import pytest
-import u_boot_utils
+import utils
 
 @pytest.mark.buildconfigspec('cmd_pinmux')
 def test_pinmux_usage_1(ubman):