convert flowDb to class and added test cases.

packages/mermaid/src/diagrams/class/classDiagram.ts

@@ -1,13 +1,15 @@
 import type { DiagramDefinition } from '../../diagram-api/types.js';
 // @ts-ignore: JISON doesn't support types
 import parser from './parser/classDiagram.jison';
-import db from './classDb.js';
+import { ClassDB } from './classDb.js';
 import styles from './styles.js';
 import renderer from './classRenderer-v3-unified.js';
 
 export const diagram: DiagramDefinition = {
   parser,
-  db,
+  get db() {
+    return new ClassDB();
+  },
   renderer,
   styles,
   init: (cnf) => {
@@ -15,6 +17,5 @@ export const diagram: DiagramDefinition = {
       cnf.class = {};
     }
     cnf.class.arrowMarkerAbsolute = cnf.arrowMarkerAbsolute;
-    db.clear();
   },
 };