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.
Merged from http://go/wvgerrit/132883
Test: duration_use_case_test and nonce flood tests on bonito
Bug: 198329759
Change-Id: I2d66032a88dd0996252f1c31aee250246e6868fe
3ca173fa16