Shared loading and running pool (#1161)

Change the behavior of the concurrency setting so that loading and running
  don't have separate pools.
  * The loading and running of a test are now done with the same resource, and
    the concurrency setting uniformly affects each. With `-j1` only a single
    test will ever be loaded at a time.
  * Previously the loading pool was 2x larger than the actual concurrency
    setting which could cause flaky tests due to tests being loaded while
    other tests were running, even with `-j1`.