testsuite: Tweak T6048 bounds

Summary:
This should fix the Harbormaster builds; the bounds are just a little too narrow.

Signed-off-by: Austin Seipp <austin@well-typed.com>

Test Plan: Harbormaster should build successfully.

Reviewers: simonmar

Subscribers: phaskell, simonmar, relrod, carter

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D65