Fix reference leak hunting in regrtest: compute also deltas (of
reference count and file descriptor count) during warmup, to ensure
that everything is initialized before starting to hunt reference
leaks.

Other changes:

* Replace gc.collect() with support.gc_collect() in clear_caches()
* dash_R() is now more quiet with --quiet option (don't display
  progress).
* Precompute the full range for "for it in range(repcount):" to
  ensure that the iteration doesn't allocate anything new.
* dash_R() now is responsible to call warm_caches().

(cherry picked from commit 5aaac94e)