testsuite: Add regression test for #23861

Simon says this was fixed by

commit 8d686854
Author: sheaf <sam.derbyshire@gmail.com>
Date:   Fri Aug 4 15:28:45 2023 +0200

    Remove zonk in tcVTA

testsuite/tests/typecheck/should_compile/T23861.hs

+module M where
+
+newtype GetDiscardingUnlift a = MkGetDiscardingUnlift
+    { unGetDiscardingUnlift :: forall m. Either a m
+    }
+
+build :: forall a. a -> GetDiscardingUnlift a
+build w =
+    case build w of
+        MkGetDiscardingUnlift getDiscardingUnlift' ->
+         let getDiscardingUnlift'' :: forall m. Either a m
+             getDiscardingUnlift'' = getDiscardingUnlift' @m
+         in  MkGetDiscardingUnlift getDiscardingUnlift''

testsuite/tests/typecheck/should_compile/all.T

@@ -891,3 +891,4 @@ test('T18986b', normal, compile, [''])
 test('T23413', normal, compile, [''])
 test('TcIncompleteRecSel', normal, compile, ['-Wincomplete-record-selectors'])
 test('InstanceWarnings', normal, multimod_compile, ['InstanceWarnings', ''])
+test('T23861', normal, compile, [''])