chore: add failing grind tests about decide

tests/lean/grind/decide.lean

@@ -0,0 +1,21 @@
+---
+
+example {P Q : Prop} [Decidable P] [Decidable Q] : (decide P || decide Q) = decide (P ∨ Q) := by grind
+
+---
+
+@[grind] theorem eq_head_or_mem_tail_of_mem_cons {a b : α} {l : List α} :
+    a ∈ b :: l → a = b ∨ a ∈ l := List.mem_cons.mp
+
+attribute [grind] List.mem_cons_self, List.mem_cons_of_mem
+
+-- This succeeds:
+example [DecidableEq α] {l : List α} :
+    (y ∈ a :: l) = (y = a) ∨ y ∈ l := by
+  grind
+
+
+-- but inserting some `decide`s fails:
+example [BEq α] [LawfulBEq α] {l : List α} :
+    decide (y ∈ a :: l) = (y == a || decide (y ∈ l)) := by
+  grind