testsuite: Add broken test for #15289

The stderr output is merely a guess at what we should expect, but currently this
is certainly broken.

testsuite/tests/patsyn/should_fail/T15289.hs

+{-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+module Oops where
+
+pattern What = True :: Maybe

testsuite/tests/patsyn/should_fail/T15289.stderr

+  T15289.hs:5:16: error:
+      • Couldn't match expected type ‘Maybe’ with actual type ‘Bool’
+      • In the pattern: True
+        In the pattern: True :: Maybe
+        In the declaration for pattern synonym ‘What’
+    |
+  5 | pattern What = True :: Maybe
+    |                ^^^^

testsuite/tests/patsyn/should_fail/all.T

@@ -42,3 +42,4 @@ test('T14380', normal, compile_fail, [''])
 test('T14498', normal, compile_fail, [''])
 test('T14552', normal, compile_fail, [''])
 test('T14507', normal, compile_fail, ['-dsuppress-uniques'])
+test('T15289', expect_broken(15289), compile_fail, [''])