Use branch for ms testing suite.

tester.py

@@ -253,6 +253,9 @@ class Tester():
             if os.path.exists(ms_path):
                 log('Updating ms-testing-suite in "%s".' % ms_path)
                 os.chdir(ms_path)
+                branch = utils.get_conf('ENVSETUP_BRANCH') or 'stable'
+                if branch:
+                    subprocess.call(['git', 'checkout', branch])
                 subprocess.call(['git', 'pull', '--recurse-submodules'])
                 subprocess.call(['git', 'submodule', 'update', '--init', '--recursive'])
                 os.chdir(self.root_dir)