CodeGen: Display multiple lines better in test failure

In comparing ASCII-only strings, unittest module displays
multiple lines strings in one line using new line characters ("\n")

  "abc\nxyz" != "abo\nxyz"

With this change, it shows the lines which differs and its around.

  "abc\nxyz" != "abo\nxyz"
  - abc
  ?   ^
  + abo
  ?   ^
  xyz


Bug: 839389
Change-Id: Icd8fc5001f02d1b6d7c6abb277836d842b1f424f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1909767Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714165}

third_party/blink/renderer/bindings/scripts/bind_gen/code_node_test.py

@@ -17,6 +17,10 @@ from .mako_renderer import MakoRenderer
 
 
 class CodeNodeTest(unittest.TestCase):
+    def setUp(self):
+        super(CodeNodeTest, self).setUp()
+        self.addTypeEqualityFunc(str, self.assertMultiLineEqual)
+
     def assertRenderResult(self, node, expected):
         if node.renderer is None:
             node.set_renderer(MakoRenderer())