dhall-lsp-server: Fix doctests (#1505)

Author: sjakobi

appveyor.yml

@@ -58,8 +58,7 @@ for:
     - chcp 65001 && stack test dhall
     - stack test dhall-json
     - stack test dhall-bash
-    # - stack test dhall-lsp-server:doctest # Disabled while the tests are broken.
-    - stack test dhall-lsp-server:tests
+    - stack test dhall-lsp-server
     - stack bench dhall --benchmark-arguments "--quick --min-duration=0 --include-first-iter"
 
 -

dhall-lsp-server/dhall-lsp-server.cabal

@@ -98,7 +98,8 @@ Test-Suite doctest
         base                           ,
         directory  >= 1.3.1.5 && < 1.4 ,
         filepath                 < 1.5 ,
-        doctest    >= 0.7.0   && < 0.17
+        doctest    >= 0.7.0   && < 0.17,
+        QuickCheck
     Other-Extensions: OverloadedStrings RecordWildCards
     Default-Language: Haskell2010
     -- `doctest` doesn't work with `MIN_VERSION` macros before GHC 8