Properly call 'PrepareFunctionForOptimization' in mjsunit test

Adding a 'PrepareFunctionForOptimization' call will prevent the test
case in question to become flaky when stress testing bytecode flushing.

R=jarin@chromium.org

Bug: v8:9123
Change-Id: If192ebf571d3cd4f0d1ee31bc3f6313b74d3c866
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1564202
Auto-Submit: Simon Zünd <szuend@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60803}

test/mjsunit/es6/block-let-crankshaft-sloppy.js

@@ -467,6 +467,9 @@ function g(x) {
   }
 }
 
+%PrepareFunctionForOptimization(f);
+%PrepareFunctionForOptimization(g);
+
 for (var i=0; i<10; i++) {
   f(i);
   g(i);