Slightly improve tests that rely on lazy compilation.

R=rossberg@chromium.org
BUG=v8:3712
LOG=N

Review URL: https://codereview.chromium.org/743843003

Cr-Commit-Position: refs/heads/master@{#25463}

test/mjsunit/function-length-accessor.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --harmony-scoping
+// Flags: --harmony-scoping --lazy
 
 function foo(a, b, c, d) {
   "use strict"

test/mjsunit/regress/regress-2506.js

@@ -46,7 +46,7 @@ for (const x in [1,2,3]) {
 }
 assertEquals("012", s);
 
-assertThrows(function() { for(const x in [1,2,3]) { x++ } }, SyntaxError);
+assertThrows("'use strict'; for (const x in [1,2,3]) { x++ }", SyntaxError);
 
 // Function scope
 (function() {