Fix flaky parallel recompilation test.

test/mjsunit/regress/regress-embedded-cons-string.js

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 // Flags: --fold-constants --nodead-code-elimination
 // Flags: --expose-gc --allow-natives-syntax
-// Flags: --concurrent-recompilation --concurrent-recompilation-delay=300
+// Flags: --concurrent-recompilation --concurrent-recompilation-delay=600
 
 if (!%IsConcurrentRecompilationSupported()) {
   print("Concurrent recompilation is disabled. Skipping this test.");
   quit();
 }
 
 function test(fun) {
   fun();
   fun();
   // Mark for concurrent optimization.
@@ skipping 18 matching lines @@
   return "abcdefghijklmn\u2603" + "123456789";
 }
 
 function h() {
   return "abcdefghijklmn\u2603" + "123456789\u2604";
 }
 
 test(f);
 test(g);
 test(h);