Create a new depot_tools_testing_lib to move utility modules there

tests/checkout_test.py

Index: tests/checkout_test.py
diff --git a/tests/checkout_test.py b/tests/checkout_test.py
index 5de734a0088ac597770c73ed804ce2cb77375d0e..894950fc0540d697616db9ca2dcc4c311feb45f3 100755
--- a/tests/checkout_test.py
+++ b/tests/checkout_test.py
@@ -16,11 +16,12 @@ from xml.etree import ElementTree
 ROOT_DIR = os.path.dirname(os.path.abspath(__file__))
 sys.path.insert(0, os.path.dirname(ROOT_DIR))
 
+from depot_tools_testing_lib import fake_repos
+from depot_tools_testing_lib.patches_data import GIT, RAW
+
 import checkout
 import patch
 import subprocess2
-from tests import fake_repos
-from tests.patches_data import GIT, RAW
 
 
 # pass -v to enable it.