Add test suite for #8757

testsuite/tests/patsyn/should_fail/local.hs

+{-# LANGUAGE PatternSynonyms #-}
+module ShouldFail where
+
+varWithLocalPatSyn x = case x of
+    P -> ()
+  where
+    pattern P = ()

testsuite/tests/patsyn/should_fail/local.stderr

+
+local.hs:7:5:
+    Illegal pattern synonym declaration
+      Pattern synonym declarations are only valid in the top-level scope