call app.close() afterEach in Fastify e2e example

Otherwise you'd get:
> Jest did not exit one second after the test run has completed.
> This usually means that there are asynchronous operations that weren't stopped in your tests. Consider running Jest with `--detectOpenHandles` to troubleshoot this issue.

Author: DanielKucal

content/fundamentals/unit-testing.md

@@ -287,6 +287,10 @@ describe('Cats', () => {
 >   await app.init();
 >   await app.getHttpAdapter().getInstance().ready();
 > });
+> 
+> afterEach(async () => {
+>   await app.close();
+> });
 >
 > it(`/GET cats`, () => {
 >   return app