docs(docs-infra): attemp to fix flaky mermaid test (#59994)

This test has 2 async parts, loading the markdown file and the marked parsing.
First attempt to fix the flakiness is by inline the markdown file.

If this test timeouts again in the future it means the timeout is related to marked itself (and the dynamic import of mermaid).

PR Close #59994

adev/shared-docs/pipeline/guides/testing/mermaid/mermaid.md

@@ -1,14 +0,0 @@
-```mermaid
-    graph TD;
-      A-->B;
-      A-->C;
-      B-->D;
-      C-->D;
-```
-
-```mermaid
-  pie title Pets adopted by volunteers
-      "Dogs" : 386
-      "Cats" : 85
-      "Rats" : 15
-```

adev/shared-docs/pipeline/guides/testing/mermaid/mermaid.spec.ts

@@ -20,9 +20,24 @@ describe('markdown to html', () => {
     // Extend the timeout interval tyo 15 seconds because we were seeing issues with not being able to run marked
     // within the default timeframe.
     jasmine.DEFAULT_TIMEOUT_INTERVAL = 15000;
-    const markdownContent = await readFile(runfiles.resolvePackageRelative('./mermaid.md'), {
-      encoding: 'utf-8',
-    });
+
+    // This test was flaky, 1st attemp to fix it is by inlining the markdown content
+    const markdownContent = `
+\`\`\`mermaid
+    graph TD;
+      A-->B;
+      A-->C;
+      B-->D;
+      C-->D;
+\`\`\`
+
+\`\`\`mermaid
+  pie title Pets adopted by volunteers
+      "Dogs" : 386
+      "Cats" : 85
+      "Rats" : 15
+\`\`\`
+    `;
 
     marked.use({
       async: true,