In the CompletionTest, the check for the completion proposals waits for
the full list of proposal items to be calculated. The condition is never
fulfilled because it checks for the first proposal item to change while
only its data will be replaced. In consequence, waiting for the
condition always runs into a timeout.

This change corrects the conditions and refactors the logic for checking
for the "computing" entry into a separate method.

Contributes to
https://github.com/eclipse-platform/eclipse.platform.ui/issues/906 and
https://github.com/eclipse-platform/eclipse.platform.ui/issues/907