Sync fake clock for nonce tests

Because the fake clock is getting out of sync with the real clock for
the nonce flood tests, add a call to TestSleep::SyncFakeClock() in
Clock::GetCurrentTime() to force a sync.

Bug: 198329759

util/test/test_clock.cpp

@@ -35,6 +35,7 @@ FakeClock* g_fake_clock = nullptr;
 
 // On devices running a fake OEMCrypto, we can use a fake sleep and fake time.
 int64_t Clock::GetCurrentTime() {
+  wvcdm::TestSleep::SyncFakeClock();
   if (g_fake_clock == nullptr) g_fake_clock = new FakeClock();
   return g_fake_clock->now() / 1000;
 }