Make test unthreaded so other tests don't interfere with heap size

R=hpayer@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1086423003

Cr-Commit-Position: refs/heads/master@{#27890}

test/cctest/test-api.cc

@@ -6562,7 +6562,7 @@ static void IndependentWeakHandle(bool global_gc, bool interlinked) {
 }
 
 
-THREADED_TEST(IndependentWeakHandle) {
+TEST(IndependentWeakHandle) {
   IndependentWeakHandle(false, false);
   IndependentWeakHandle(false, true);
   IndependentWeakHandle(true, false);