Add a test for AxiomInstCo

[zliu41]

I don't think we have a test case for this part of the code

Plugins.Cast from0 Plugins.AxiomInstCo {}
  | Plugins.isPredTy (Plugins.exprType from0) -> do
      inlined <- flip loopM from0 $ \from -> do
        ( \case
            Plugins.Cast from' _ ->
              if Plugins.isPredTy (Plugins.exprType from')
                then Left from'
                else Right from'
            other -> Left other
          )
          <$> (simplifyFun dflags logger [] =<\< mkInline from)
      categorifyLambda name
        =<\< simplifyFun dflags logger [] (Plugins.mkCoreApps inlined args)

outside of Heavisoft.