chore: mockfs broken on node v20.5+

Implementing the workaround mentioned in the linked issue.

re tschaub/mock-fs#377

src/fixtures.ts

@@ -51,7 +51,7 @@ function loadLiterateSource(directory: string, literateFileName: string) {
     // This couldn't really happen in practice, but do the check anyway
     throw new Error(`Sample uses literate source ${literateFileName}, but not found: ${fullPath}`);
   }
-  return fs.readFileSync(fullPath, { encoding: 'utf-8' });
+  return fs.readFileSync(fullPath).toString('utf-8');
 }
 
 /**