Try to get `cabal-testsuite` to pin down the `transformers` package-id

This will hopefully fix the recent testsuite failures in

 - `PackageTests/TestSuiteTests/ExeV10/cabal-with-hpc.multitest.hs`

due to multiple `transformer` versions clashing

cabal-testsuite/cabal-testsuite.cabal

@@ -67,6 +67,7 @@ executable cabal-tests
     process,
     optparse-applicative,
     cabal-testsuite,
+    transformers,
     exceptions
   default-language: Haskell2010