The test cases for completion proposals in CompletionTest depend on
specific timing behavior of a test proposal processor. On one hand, this
is prone to errors if proper timing is not given on the test
environment. On the other hand, the tests run unnecessarily long because
of waiting long enough.

This change replaces the fixed-time waiting of the
LongRunningBarContentAssistProcessor with an explicit trigger that
finalizes the content calculation as soon as some barrier is reached
within the tests. In addition, the long-running processor is only
enabled on demand and will be finalized whenever a test case was
executed to avoid that the processor thread is leaking to the next test
case.

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