The test method JobTest#testSetProgressGroup mixes up at least five test
cases into one method. This makes it hard to comprehend what actually is
tested and to identify the reasons for a failure. This change splits up
the five test cases into separate methods with names reflecting their
purpose and a reduced setup to what is necessary for the test case.
Since the original test method is randomly failing, this change will
make it easier to find the reasons.

Contributes to
https://github.com/eclipse-platform/eclipse.platform/issues/1026