Add test for bad method variant

Author: Noblix

tests/method-variant-bad.slang

@@ -0,0 +1,17 @@
+method sumn(n: Int): Int
+    requires n >= 0
+    ensures result == n * (n + 1)/2
+    decreases n
+{
+    var ret: Int;
+    match {
+        n == 0 => 
+            ret := 0,
+        true => 
+            var res: Int;
+            // @CheckError
+            res := sumn(n + 1);
+            ret := res + n
+    };
+    return ret
+}