Make testsuite driver Python 2.6 compatible again

Another bug in the #10164 series. Only Python 2.7 and up allow you to
omit the positional argument specifiers in format strings.

Test Plan: this fixes the Solaris builders

Reviewed By: kgardas

Differential Revision: https://phabricator.haskell.org/D750

GHC Trac Issues: #10164

(cherry picked from commit 0f03a843)