[TEST] Randomize number of available processors

We configure the threadpools according to the number of processors which is
different on every machine. Yet, we had some test failures related to this
and #6174 that only happened reproducibly on a node with 1 available processor.
This commit does:
  * sometimes randomize the number of available processors
  * if we don't randomize we should set the actual number of available processors
    in the settings on the test node
  * always print out the num of processors when a test fails to make sure we can
    reproduce the thread pool settings with the reproduce info line

Closes #6176