Support running test_that via a symlink

BUG=b:70390932
TEST=Manually patch test_that.py in lab-tools checkout; test_that works.

Change-Id: I5eceebae232d8e791ddd6380c4a419bae1ed94cb
diff --git a/site_utils/test_that.py b/site_utils/test_that.py
index d7c5f3e..99c4ad4 100755
--- a/site_utils/test_that.py
+++ b/site_utils/test_that.py
@@ -311,8 +311,10 @@
     @param argv: Script command line arguments.
     @param arguments: Parsed command line arguments.
     """
-    autotest_path = os.path.realpath(os.path.join(os.path.dirname(__file__),
-                                                  '..'))
+    autotest_path = os.path.realpath(os.path.join(
+            os.path.dirname(os.path.realpath(__file__)),
+            '..',
+    ))
     command = [os.path.join(autotest_path, 'site_utils',
                             'run_suite.py'),
                '--board=%s' % (arguments.board,),