binman: Move capture_sys_output() to test_util

This function is useful in various tests. Move it into the common test
utility module.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/image_test.py b/tools/binman/image_test.py
index 45dd237..3775e1a 100644
--- a/tools/binman/image_test.py
+++ b/tools/binman/image_test.py
@@ -7,7 +7,7 @@
 import unittest
 
 from image import Image
-from elf_test import capture_sys_output
+from test_util import capture_sys_output
 
 class TestImage(unittest.TestCase):
     def testInvalidFormat(self):