Fix parsing of maximum residency in runstdtest

Summary:
`runstdtest` switched from `-S` to `-s` output a while ago.
That broke parsing of maximum and average residency numbers.

This commit makes sure that at least maximum residency is parsed
correctly from `-s` output, while leaving the logic for `-S` output.

Reviewers: simonmar, bgamari, simonpj, osa1, AndreasK, O26 nofib

Reviewed By: bgamari

GHC Trac Issues: #16003

Differential Revision: https://phabricator.haskell.org/D5418