chore(contented): add more test coverage (#625)

#### What this PR does / why we need it:

Add more scenarios to increase test coverage.

packages/contented-pipeline-md/fixtures/Features.md

@@ -0,0 +1,14 @@
+---
+title: 'Title Override'
+description: 'Description Override'
+---
+
+# Features
+
+```mermaid
+graph TD;
+  A-->B;
+  A-->C;
+  B-->D;
+  C-->D;
+```

packages/contented-pipeline-md/src/MarkdownPipeline.unit.ts

@@ -75,4 +75,23 @@ describe('Without Config', () => {
       },
     ]);
   });
+
+  it('should process Features.md', async () => {
+    const content = await pipeline.process(rootPath, 'Features.md');
+    expect(content).toStrictEqual([
+      {
+        type: 'Markdown',
+        fields: {
+          description: 'Description Override',
+          title: 'Title Override',
+        },
+        headings: expect.any(Array),
+        path: '/features',
+        sections: [],
+        fileId: expect.stringMatching(/[0-f]{64}/),
+        modifiedDate: expect.any(Number),
+        html: '<nav class="toc"><ol class="toc-level toc-level-1"><li class="toc-item toc-item-h1"><a class="toc-link toc-link-h1" href="#features">Features</a></li></ol></nav><h1 id="features"><a aria-hidden="true" tabindex="-1" href="#features"><span class="icon icon-link"></span></a>Features</h1>\n<div class="mermaid">graph TD;\n  A-->B;\n  A-->C;\n  B-->D;\n  C-->D;\n</div>',
+      },
+    ]);
+  });
 });