`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