binman: Correct some import statements

Some of these were not converted when binman moved to use absolute paths.
Fix them.

Also drop the import of 'test' which is a directory, not a module.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/image_test.py b/tools/binman/image_test.py
index f85c3c5..e351fa8 100644
--- a/tools/binman/image_test.py
+++ b/tools/binman/image_test.py
@@ -6,7 +6,7 @@
 
 import unittest
 
-from image import Image
+from binman.image import Image
 from patman.test_util import capture_sys_output
 
 class TestImage(unittest.TestCase):