bpo-36560: Fix reference leak hunting in regrtest (GH-12744)

bpo-36560: Fix reference leak hunting in regrtest (GH-12744)

Fix reference leak hunting in regrtest: compute also deltas (of
reference count, allocated memory blocks, 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()
* Move calls to read memory statistics from dash_R_cleanup() to
  dash_R()
* Pass regrtest 'ns' to dash_R()
* 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().