test: test missing cases error

This PR tests the missing cases error.

I thought I broke this, but it seems I did not (or at least not this
way, maybe there is a way to trigger it).

tests/lean/run/matchMissingCase.lean

@@ -0,0 +1,24 @@
+inductive Enum where | a | b | c | d
+
+/--
+error: Missing cases:
+Enum.d
+Enum.c
+-/
+#guard_msgs in
+def test : Enum → Nat
+  | .a => 0
+  | .b => 0
+
+-- set_option trace.Meta.Match.match true
+
+/--
+error: Missing cases:
+Enum.d, false
+Enum.c, false
+-/
+#guard_msgs(pass trace, all) in
+def test2 : Enum → Bool → Nat
+  | .a, _ => 0
+  | .b, _ => 0
+  | _, true => 0