allow fixture file as source to content linter (#52631)

Co-authored-by: Kevin Heis <[email protected]>

Author: rachmari

src/content-linter/scripts/lint-content.js

@@ -323,7 +323,7 @@ function getFilesToLint(paths) {
         fileList.yml.push(...walkFiles(absPath, ['.yml']))
       }
     } else {
-      if (isInDir(absPath, contentDir)) {
+      if (isInDir(absPath, contentDir) || isAFixtureMdFile(absPath)) {
         fileList.content.push(absPath)
       } else if (isInDir(absPath, dataDir)) {
         if (absPath.endsWith('.yml')) {
@@ -691,3 +691,7 @@ function isOptionsValid() {
   }
   return true
 }
+
+function isAFixtureMdFile(filePath) {
+  return filePath.includes('/src') && filePath.includes('/fixtures') && filePath.endsWith('.md')
+}