Fix acting as null user not reset existing user in test

[nanaya]

There's this test but it's testing test helper so I'm not sure if it makes sense testing it? 🤔 Can jam it somewhere... (where?)

    public function testWat()
    {
        $user = User::factory()->create();
        $this->actAsScopedUser($user, ['public']);
        $this->assertSame(\Auth::user()->getKey(), $user->getKey());

        $this->actAsScopedUser(null, ['public']);
        $this->assertNull(\Auth::user());
    }