Add test cases for #3668

Previous commits fix the issue.

Closes #3668

Author: catamorphism

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/run-pass/issue-3668.rs renamed to src/test/compile-fail/issue-3668.rs

@@ -1,12 +1,11 @@
-// xfail-test
 struct P { child: Option<@mut P> }
 trait PTrait {
    fn getChildOption() -> Option<@P>;
 }
 
 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;
    }
 }