* Used advanced Shell activation wait method from SWT
* Replaced incorrect `Shell.setVisible()` with `Shell.open()`
  Because the former doesn't try to activate Shell
* Screenshot is only made when test fails

For debugging of https://github.com/eclipse-platform/eclipse.platform.ui/issues/910

Signed-off-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>