Fuchsia: Use low-level prng call, add test, update test runner.

runtime/bin/crypto_test.cc

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include "bin/crypto.h"
+#include "platform/assert.h"
+#include "platform/globals.h"
+#include "vm/unit_test.h"
+
+namespace dart {
+namespace bin {
+
+TEST_CASE(GetRandomBytes) {
+  const intptr_t kNumRandomBytes = 127;
+  uint8_t buf[kNumRandomBytes];
+  const bool res = Crypto::GetRandomBytes(kNumRandomBytes, buf);
+  EXPECT(res);
+}
+
+}  // namespace bin
+}  // namespace dart