Look for "unstable feature" error code in test

Conditionals and loops now have unstable features, and `feature_err` has
its own error code. I think that `feature_err` should take an error code
as a parameter, but don't have the energy to make this change throughout
the codebase. Also, the error code system may be torn out entirely.

src/librustc_error_codes/error_codes/E0744.md

@@ -3,7 +3,7 @@ Control-flow expressions are not allowed inside a const context.
 At the moment, `if` and `match`, as well as the looping constructs `for`,
 `while`, and `loop`, are forbidden inside a `const`, `static`, or `const fn`.
 
-```compile_fail,E0744
+```compile_fail,E0658
 const _: i32 = {
     let mut x = 0;
     loop {