fix: clear observers after each test run (#519)

Author: thebuilder

src/observe.ts

@@ -1,6 +1,6 @@
 import { ObserverInstanceCallback } from './index';
 
-const ObserverMap = new Map<
+export const ObserverMap = new Map<
   string,
   {
     id: string;

src/test-utils.ts

@@ -1,4 +1,5 @@
 import { act } from 'react-dom/test-utils';
+import { ObserverMap } from './observe';
 
 type Item = {
   callback: IntersectionObserverCallback;
@@ -48,6 +49,7 @@ afterEach(() => {
   // @ts-ignore
   global.IntersectionObserver.mockClear();
   observers.clear();
+  ObserverMap.clear();
 });
 
 function triggerIntersection(