Add a test for #5332 (unboxed singleton tuples and TH)

testsuite/tests/ghc-regress/th/TH_unboxedSingleton.hs

+{-# LANGUAGE TemplateHaskell, UnboxedTuples #-}
+
+module TH_unboxedSingleton where
+
+f :: () -> (# Int #)
+f () = $( [| (# 3 #) |] )
+

testsuite/tests/ghc-regress/th/all.T

@@ -181,3 +181,5 @@ test('T4949', normal, compile, ['-v0'])
 test('T5126', normal, compile, ['-v0'])
 test('T5217', normal, compile, ['-v0 -dsuppress-uniques -ddump-splices'])
 test('T5037', normal, compile, ['-v0'])
+test('TH_unboxedSingleton', normal, compile, ['-v0'])
+