markdown-tests: Allow ignoring certain fixtures while developing.

Usually, to debug a small change, you have to remove some tests from JSON
because of lack of support for comments in JSON. This commit allows to
ignore some tests by setting `"ignore" : true` in the bugdown fixtures.

Also, since this is only for while developing, the complete test suite will
throw an error if we leave an 'ignored' test in a commit.

docs/subsystems/markdown.md

@@ -73,6 +73,14 @@ This procedure prevents any server-side rendering.  If you don't do
 this, backend will likely render the Markdown you're testing and swap
 it in before you can see the frontend's rendering.
 
+If you are working on a feature that breaks multiple testcases, and want
+to debug the testcases one by one, you can add `"ignore": true` to any
+testcases in `markdown_test_cases.json` that you want to ignore. This
+is a workaround due to lack of comments support in JSON. Revert your
+"ignore" changes before committing. After this, you can run the frontend
+tests with `tools/test-js-with-node markdown` and backend tests with
+`tools/test-backend zerver.tests.test_bugdown.BugdownTest.test_bugdown_fixtures`.
+
 ## Changing Zulip's markdown processor
 
 First, you will likely find these third-party resources helpful:

frontend_tests/node_tests/markdown.js

@@ -180,6 +180,12 @@ var bugdown_data = JSON.parse(fs.readFileSync(path.join(__dirname, '../../zerver
     var tests = bugdown_data.regular_tests;
 
     tests.forEach(function (test) {
+
+        // Ignore tests if specified
+        if (test.ignore === true) {
+            return;
+        }
+
         var message = {raw_content: test.input};
         page_params.translate_emoticons = test.translate_emoticons || false;
         markdown.apply_markdown(message);

zerver/tests/test_bugdown.py

@@ -242,18 +242,30 @@ class BugdownTest(ZulipTestCase):
 
         return test_fixtures, data['linkify_tests']
 
+    def test_bugdown_no_ignores(self) -> None:
+        # We do not want any ignored tests to be committed and merged.
+        format_tests, linkify_tests = self.load_bugdown_tests()
+        for name, test in format_tests.items():
+            message = 'Test "%s" shouldn\'t be ignored.' % (name,)
+            is_ignored = test.get('ignore', False)
+            self.assertFalse(is_ignored, message)
+
     @slow("Aggregate of runs dozens of individual markdown tests")
     def test_bugdown_fixtures(self) -> None:
         format_tests, linkify_tests = self.load_bugdown_tests()
         valid_keys = set(["name", "input", "expected_output",
                           "backend_only_rendering",
                           "marked_expected_output", "text_content",
-                          "translate_emoticons"])
+                          "translate_emoticons", "ignore"])
 
         for name, test in format_tests.items():
             # Check that there aren't any unexpected keys as those are often typos
             self.assertEqual(len(set(test.keys()) - valid_keys), 0)
 
+            # Ignore tests if specified
+            if test.get('ignore', False):
+                return  # nocoverage
+
             if test.get('translate_emoticons', False):
                 # Create a userprofile and send message with it.
                 user_profile = self.example_user('othello')