Add applicative import for old GHCs in BuildToolsPathForeign test

Should we stick this in the testing prelude too?

cabal-testsuite/PackageTests/BuildToolsPathForeign/setup.test.hs

 import Test.Cabal.Prelude
 
+import Control.Applicative ((<$>))
 import Control.Monad.IO.Class
 import System.Environment