process_test unit test behaves differently on CI and local machine

https://github.com/envoyproxy/nighthawk/blob/e6f9af70d446ee018f1d72f8e46b8c1a22c186f4/test/process_test.cc#L279

With `TimestampToNanoseconds`, passes on CI but fails on my machine with error:
```
test/process_test.cc:278: Failure
Expected equality of these values:
Envoy::ProtobufUtil::TimeUtil::TimestampToNanoseconds( output.results()[0].execution_start())
Which is: 1610660382038451000
options_->scheduled_start().value().time_since_epoch().count()
Which is: 1610660382038451
```

With `TimestampToMicroseconds`, passes on my machine but fails on CI, with a similar error.