Cleanup func-test suite

This test-suite is vastly legacy from Haskell IDE Engine.
It was originally written before we had a plugin-based architecture
where each plugin tests its feature in isolation.
Over time, this test-suite bitrotted, where a lot of testcases were
either redundant or out-of-date and consequentially disabled.

We clean up the test-suite, delete old tests and remove unused testdata.

haskell-language-server.cabal

@@ -529,21 +529,12 @@ test-suite func-test
 
   main-is:            Main.hs
   other-modules:
-    Command
-    Completion
     Config
-    Deferred
-    Definition
-    Diagnostic
     Format
     FunctionalBadProject
     FunctionalCodeAction
     HieBios
-    Highlight
     Progress
-    Reference
-    Symbol
-    TypeDefinition
     Test.Hls.Command
     Test.Hls.Flags