Add tests for `simpl` on `last`.

tests/list.ref

+1 goal
+  
+  ============================
+  None = None
+1 goal
+  
+  ============================
+  Some 10 = Some 10
+1 goal
+  
+  ============================
+  Some 11 = Some 11
+1 goal
+  
+  l : list nat
+  ============================
+  last (11 :: l) = last (11 :: l)
+1 goal
+  
+  l : list nat
+  ============================
+  last (10 :: l) = last (10 :: l)

tests/list.v

+From stdpp Require Import list.
+
+Lemma last_simpl_test_nil : last [] =@{option nat} None.
+Proof. simpl. Show. done. Qed.
+
+Lemma last_simpl_test_singleton : last [10] = Some 10.
+Proof. simpl. Show. done. Qed.
+
+Lemma last_simpl_test_double : last [10; 11] = Some 11.
+Proof. simpl. Show. done. Qed.
+
+Lemma last_simpl_test_cons_cons l : last (10 :: 11 :: l) = last (11 :: l).
+Proof. simpl. Show. done. Qed.
+
+(* The following should not [simpl] and result in a [match]. *)
+Lemma last_simpl_test_cons l : last (10 :: l) = last (10 :: l).
+Proof. simpl. Show. done. Qed.