Added assertions in runTest() to find out who disposes the display
before the test application finishes.

Fixed few tests that didn't created Display but disposed it.

Note: testingFinished() might or might not run before the display is
disposed, because it runs in parallel to the application that closes
itself on UI thread after runTest() left syncExec() call. So checking
for disposed display in testingFinished() does not make sense.

See https://github.com/eclipse-platform/eclipse.platform.ui/issues/50