testsuite: Add test for #12024

Test Plan: Validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2702

GHC Trac Issues: #12024

testsuite/tests/ghci/scripts/T12024.script

+:set -XPatternSynonyms -XGADTs
+data A a where X :: A [x]
+
+pattern X' = X
+
+-- This should render the equality with ~ despite the pattern internally using
+-- GHC.Prim.~#.
+:info X'

testsuite/tests/ghci/scripts/T12024.stdout

+pattern X' :: () => a ~ [x] => A a 	-- Defined at <interactive>:4:1

testsuite/tests/ghci/scripts/all.T

@@ -265,3 +265,4 @@ test('T12091',
      [expect_broken(12091), extra_run_opts('-fobject-code')],
      ghci_script, ['T12091.script'])
 test('T12523', normal, ghci_script, ['T12523.script'])
+test('T12024', normal, ghci_script, ['T12024.script'])