If a test suspends the JobManager and then waits for the build job to
finish then an exception will be thrown. The reason for this is that
waiting for a build job while the JobManager is not running is an
endless wait so the programmer should be informed.

Let the ResourceTest class restore the original state of the JobManager
in the tearDown method in order to minimize the chances that a test
throws such an exception just because the test did not clean up after
its execution (print a warning if the test did not clean up after
itself).

Contributes to
https://github.com/eclipse-platform/eclipse.platform/issues/626