gh-75876: Report the peak memory use of a bigmem test (GH-156024)

In verbose mode, report how much memory the test really used, next to how
much it declared.  The subprocess measures its own peak; if it died before
reporting it, the parent reports what it sampled, as a lower bound.

Report also the number of major page faults, if any: the test did not fit in
memory, so its timing means little.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>