Tighten up Stan plugin language extension test cases

These changes ensure that the tests will fail given bad mappings in
either the `cabalExtensionsMap` OR the `checksMap`. Either of these
could cause bad behavior as seen in issue haskell#3174.

plugins/hls-stan-plugin/test/Main.hs

@@ -44,13 +44,17 @@ tests =
         runStanSession "" $ do
           doc <- openDoc "extension-tests/language-pragma/LanguagePragmaTest.hs" "haskell"
           diags <- waitForDiagnosticsFromSource doc "stan"
-          liftIO $ length diags @?= 0
+          -- We must include at least one valid diagnostic in our test file to avoid
+          -- the false-positive case where Stan finds no analyses to perform due to a
+          -- bad mapping, which would also lead to zero diagnostics being returned.
+          liftIO $ length diags @?= 1
           return ()
     , testCase "respects language extensions defined in the .cabal file" $
         runStanSession "" $ do
           doc <- openDoc "extension-tests/cabal-file/CabalFileTest.hs" "haskell"
           diags <- waitForDiagnosticsFromSource doc "stan"
-          liftIO $ length diags @?= 0
+          -- We need at least one valid diagnostic here too, for the same reason as above.
+          liftIO $ length diags @?= 1
           return ()
     ]
 

plugins/hls-stan-plugin/test/testdata/extension-tests/cabal-file/CabalFileTest.hs

@@ -2,3 +2,6 @@ module CabalFileTest () where
 
 -- With `StrictData` enabled in the `.cabal` file, Stan shouldn't complain here:
 data A = A Int Int
+
+-- ...but it should still complain here!
+kewlFunc = undefined

plugins/hls-stan-plugin/test/testdata/extension-tests/language-pragma/LanguagePragmaTest.hs

@@ -4,3 +4,6 @@ module LanguagePragmaTest () where
 
 -- With the above `StrictData` language pragma, Stan shouldn't complain here:
 data A = A Int Int
+
+-- ...but it should still complain here!
+kewlFunc = undefined