Fix toast test

app/soapbox/features/verification/__tests__/registration.test.tsx

@@ -75,9 +75,12 @@ describe('<Registration />', () => {
         fireEvent.submit(screen.getByTestId('button'), { preventDefault: () => {} });
       });
 
+      await waitFor(() => {
         expect(screen.getByTestId('toast')).toHaveTextContent(/this username is unavailable/i);
       });
 
+    });
+
     it('handles generic errors', async() => {
       __stub(mock => {
         mock.onPost('/api/v1/pepe/accounts').reply(500, {});