Remove unused Data.String import from tests/Main.hs

The qualified import of Data.String was redundant and caused the
-Werror CI build to fail.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jwiegley

tests/Main.hs

@@ -5,7 +5,6 @@ module Main where
 import qualified Control.Exception as Exc
 import Data.Fix
 import Data.List (isSuffixOf, lookup)
-import qualified Data.String as String
 import Data.Time
 import qualified EvalTests
 import GHC.Err (errorWithoutStackTrace)