PowerShell seems to have a hard time when a flood of async tasks is
  scheduled at the same time.
  I speculated that WaitForMultipleObjects() in Windows can only take up
  to 64 process handles and if you want to handle more than you should write
  some additional code which can be sub-optimal. I.e to implement Wait-Job -Any.
  I decided to test that suggestion and introduced a $Concurrency parameter with a default value of 62.
  So in the new version the script fires up up to $Concurrency value
  and wait for anything to complete before starting any more processes.
  This improved matters greatly. Individual tests against ramdrive now
  run in 8 minutes and all of the 200+ db_tests run in 9 minutes with concurrency
  values of 8-16. About 48 is required to load a CPU on my box running against HD
  but that does not improve running times much.

  Other changes include respect -EnableJE for the individual test exes.
  Enforce exclusions for the individual tests.