test: fix classDiagramGrammer unit test

The classDiagramGrammer.spec.ts unit test had some bad filepath
manipulation that fails on UNIX platforms.

Instead, we can use the recommended method from the Node.JS
documentation, see https://nodejs.org/api/esm.html#importmetaurl.

Fixes: 221640a

packages/mermaid/src/diagrams/class/classDiagramGrammar.spec.ts

@@ -1,10 +1,10 @@
 import { readFile } from 'node:fs/promises';
+import { fileURLToPath } from 'node:url';
 // @ts-ignore - no types
 import { LALRGenerator } from 'jison';
-import path from 'path';
 
 const getAbsolutePath = (relativePath: string) => {
-  return new URL(path.join(__dirname, relativePath)).pathname;
+  return fileURLToPath(new URL(relativePath, import.meta.url));
 };
 
 describe('class diagram grammar', function () {