[System] Fix tests that regressed in e6536dd.

[System] Fix tests that regressed in e6536dd.

* I got confused between [TestFixtureSetup] and [Setup], so fix tests whose
  [Setup] methods were changed/removed to properly clean up after each test
  (using a [TearDown] method) so that the next time the on-demand creation of
  objects works as expected (i.e. objects created in the [Setup] method should
  be created on-demand once for each method instead of once for each class).

* Also fix SocketAcceptAsyncTest, the change to properly catch exceptions on
  background threads was incorrect.