test: add helper toDeepEqual for strict html equal

test/helpers/helpers.js

@@ -1,5 +1,6 @@
 const marked = require('../../src/marked.js');
 const htmlDiffer = require('./html-differ.js');
+const assert = require('assert').strict;
 
 beforeEach(() => {
   marked.setOptions(marked.getDefaults());
@@ -37,6 +38,21 @@ beforeEach(() => {
           return result;
         }
       };
-    }
+    },
+    toDeepEqual: () => ({
+      compare: async(spec, expected) => {
+        const result = {};
+        const actual = marked(spec.markdown, spec.options);
+        result.pass = assert.deepStrictEqual(expected, actual) === undefined;
+
+        if (result.pass) {
+          result.message = `${spec.markdown}\n------\n\nExpected: Should Fail`;
+        } else {
+          const diff = await htmlDiffer.firstDiff(actual, expected);
+          result.message = `Expected: ${diff.expected}\n  Actual: ${diff.actual}`;
+        }
+        return result;
+      }
+    })
   });
 });