Several test cases of VirtualTableViewerTest randomly fail. This
includes explicit failures of testRenameWithSorter() and testContains()
and silent failures of other test cases that simply return successfully
even if the test actually failed. The reason is a missing processing of
an update of the table data, which is most likely caused by the shell
not having focus.

With this change, the test execution ensures that in cases where an
update of the table data is expected, the shell is forced active and
events are processed until the data is finally set properly. In
addition, early returns producing silent failures are replaced by
assertions leading to explicit failures in case the condition is still
not met.

Contributes to
https://github.com/eclipse-platform/eclipse.platform.ui/issues/1583
May fix https://github.com/eclipse-platform/eclipse.platform.ui/issues/1005