Add test cases for #3668

Previous commits fix the issue.

Closes #3668

src/test/compile-fail/issue-3668-2.rs

@@ -0,0 +1,5 @@
+fn f(x:int) {
+    const child: int = x + 1; //~ ERROR attempt to use a non-constant value in a constant
+}
+
+fn main() {}

src/test/compile-fail/issue-3668.rs

@@ -1,4 +1,3 @@
-// xfail-test
 struct P { child: Option<@mut P> }
 trait PTrait {
    fn getChildOption() -> Option<@P>;
@@ -6,7 +5,7 @@ trait PTrait {
 
 impl P: PTrait {
    fn getChildOption() -> Option<@P> {
-       const childVal: @P = self.child.get();
+       const childVal: @P = self.child.get(); //~ ERROR attempt to use a non-constant value in a constant
        fail;
    }
 }

src/test/run-pass/issue-3688-2.rs

@@ -1,6 +0,0 @@
-// xfail-test
-fn f(x:int) {
-    const child: int = x + 1;
-}
-
-fn main() {}