refactor: Rename bugdown_assert to markdown_assert.

This commit is part of series of commits aimed at renaming bugdown to
markdown.

frontend_tests/node_tests/markdown.js

@@ -257,12 +257,12 @@ run_test('marked_shared', () => {
         const output = message.content;
         const error_message = `Failure in test: ${test.name}`;
         if (test.marked_expected_output) {
-            global.bugdown_assert.notEqual(test.expected_output, output, error_message);
-            global.bugdown_assert.equal(test.marked_expected_output, output, error_message);
+            global.markdown_assert.notEqual(test.expected_output, output, error_message);
+            global.markdown_assert.equal(test.marked_expected_output, output, error_message);
         } else if (test.backend_only_rendering) {
             assert.equal(markdown.contains_backend_only_syntax(test.input), true);
         } else {
-            global.bugdown_assert.equal(test.expected_output, output, error_message);
+            global.markdown_assert.equal(test.expected_output, output, error_message);
         }
     });
 });

frontend_tests/zjsunit/index.js

@@ -88,8 +88,8 @@ function short_tb(tb) {
     return lines.splice(0, i + 1).join('\n') + '\n(...)\n';
 }
 
-// Set up bugdown comparison helper
-global.bugdown_assert = require('./bugdown_assert.js');
+// Set up markdown comparison helper
+global.markdown_assert = require('./markdown_assert.js');
 
 function run_one_module(file) {
     console.info('running tests for ' + file.name);

frontend_tests/zjsunit/markdown_assert.js

@@ -1,5 +1,5 @@
 /**
- * bugdown_assert.js
+ * markdown_assert.js
  *
  * Used to determine whether two Markdown HTML strings are semantically
  * equivalent. Differs from the naive string-comparison approach in that