copilot-theorem: Look for falsifiable (not invalid). Refs Copilot-Language#495.

In Kind2-0.7.2, disproven properties are tagged with `falsifiable` in the XML
output, but the code in `copilot-theorem`'s Kind2 backend was instead searching
for a tag named `invalid`. As a result, `copilot-theorem` would error when
attempting to disprove properties that are false, as it fail to parse the XML
output. This fixes the issue by replacing `invalid` with `falsifiable`.

Author: RyanGlScott

copilot-theorem/src/Copilot/Theorem/Kind2/Output.hs

@@ -20,7 +20,7 @@ parseOutput prop xml = fromJust $ do
   root <- parseXMLDoc xml
   case findAnswer . findPropTag $ root of
     "valid"   -> return (Output Valid   [])
-    "invalid" -> return (Output Invalid [])
+    "falsifiable" -> return (Output Invalid [])
     s         -> err $ "Unrecognized status : " ++ s
 
   where