Add tests to exercise the "pattern has N field(s), but" error patterns.

src/test/compile-fail/alt-pattern-field-mismatch-2.rs

@@ -0,0 +1,16 @@
+fn main() {
+    enum color {
+        rgb(uint, uint, uint),
+        cmyk(uint, uint, uint, uint),
+        no_color,
+    }
+
+    fn foo(c: color) {
+        alt c {
+          rgb(_, _, _) { }
+          cmyk(_, _, _, _) { }
+          no_color(_) { }
+          //!^ ERROR this pattern has 1 field, but the corresponding variant has no fields
+        }
+    }
+}

src/test/compile-fail/alt-pattern-field-mismatch.rs

@@ -0,0 +1,16 @@
+fn main() {
+    enum color {
+        rgb(uint, uint, uint),
+        cmyk(uint, uint, uint, uint),
+        no_color,
+    }
+
+    fn foo(c: color) {
+        alt c {
+          rgb(_, _) { }
+          //!^ ERROR this pattern has 2 fields, but the corresponding variant has 3 fields
+          cmyk(_, _, _, _) { }
+          no_color { }
+        }
+    }
+}