Fix access for pre-script dict member

When constructing trees with the helper tool, the yaml entries are keys
in a dict rather object attributes.

Change-Id: I16b2ee4967c3f137f2b4385ec4dbc59c962ea4ef

git_upstream/tests/build-test-tree.py

@@ -46,7 +46,7 @@ def build_test_tree(basedir, filename):
                                data['branches'].values())
 
             if 'pre-script' in data:
-                output = subprocess.check_output(getattr(data, 'pre-script'),
+                output = subprocess.check_output(data['pre-script'],
                                                  stderr=subprocess.STDOUT,
                                                  shell=True)
                 print("pre-script output:\n%s" % output)