Add base bounds for test-suite

cabal 3.12+ is stricter in its checks. Our test suite doesn't depend on
xmonad-contrib (it compiles the modules itself with possibly different
preprocessor defines), thus it doesn't inherit its base bounds.

See haskell/cabal#10162

xmonad-contrib.cabal

@@ -481,7 +481,7 @@ test-suite tests
                   XMonad.Util.XUtils
                   XPrompt
   hs-source-dirs: tests, .
-  build-depends: base
+  build-depends: base >= 4.12 && < 5,
                , QuickCheck >= 2
                , X11 >= 1.10 && < 1.11
                , bytestring >= 0.10 && < 0.13