Fix rts/T9579 tests on OS X

Authored by erikd.

Description

Fix rts/T9579 tests on OS X

Sed on OS X does not understand 's/[0-9]\+ bytes/NUM bytes/g' but
sed on Linux and OS X do understand 's/[0-9]* bytes/NUM bytes/g'.

Test Plan: Run all rts/T9579 tests on Linux and Mac

Reviewers: thomie, austin, bgamari

Differential Revision: https://phabricator.haskell.org/D1394