Add some tests to check that no unnecessary de/activation of tabs is
happening and add some assertions to existing tests too. These
improvements are necessary in order to guarantee that fixing
https://github.com/eclipse-platform/eclipse.platform.swt/issues/46 do
not change the existing behavior of LaunchConfigurationTabGroupViewer.

Contributes to
https://github.com/eclipse-platform/eclipse.platform/issues/859
Contributes to
https://github.com/eclipse-platform/eclipse.platform.swt/issues/46