Add test case for CompleteP pretty-printer (#19270)

libraries/template-haskell/tests/all.T

 # difficult to test TH with profiling, because we have to build twice
 test('dataToExpQUnit', [omit_ways(prof_ways), req_interp], compile, ['-v0'])
+test('pragCompletePpr', [omit_ways(prof_ways), req_interp], compile_and_run, [''])

libraries/template-haskell/tests/pragCompletePpr.hs

+module Main (main) where
+
+import Language.Haskell.TH.Ppr    (ppr)
+import Language.Haskell.TH.Syntax (Dec (PragmaD), Pragma (CompleteP), mkName)
+
+main :: IO ()
+main = print $ ppr $ PragmaD $ CompleteP [mkName "Foo", mkName "Bar"] $ Just $ mkName "Bar"

libraries/template-haskell/tests/pragCompletePpr.stdout

+{-# COMPLETE Foo, Bar :: Bar #-}