Add GC request to libFuzzers in attempt to avoid parasitic coverage.

test/fuzzer/regexp.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
 
 #include "include/v8.h"
 #include "src/factory.h"
@@ skipping 53 matching lines @@
       i::MaybeHandle<i::JSRegExp> maybe_regexp =
           i::JSRegExp::New(source, static_cast<i::JSRegExp::Flags>(flags));
       if (!maybe_regexp.ToHandle(&regexp)) continue;
     }
     Test(isolate, regexp, one_byte, results_array);
     Test(isolate, regexp, two_byte, results_array);
     Test(isolate, regexp, factory->empty_string(), results_array);
     Test(isolate, regexp, source, results_array);
   }
 
+  isolate->RequestGarbageCollectionForTesting(
+      v8::Isolate::kFullGarbageCollection);
   return 0;
 }