Optimize the sleep time in UTs

[dangleptr]

As for these tests, I think there are two ways to make them faster. First, we could have shared the same MockServer instead of launch one per test. Second, seconds is appropriate for a polling interval, but too long for unit testing, while I cannot come up with a better way. But you still could impove it by usleep(FLAGS_load_data_interval_secs * 1000000 + 100000/*100ms*/) to save 0.9s per sleep.

Of course, we could leave these work to the next PR.

Originally posted by @dutor in #383

[dangleptr]

duration_test .................... Passed 11.04 sec
stats_manager_test ............... Passed 63.02 sec
meta_client_test ................. Passed 16.11 sec
file_based_wal_test .............. Passed 8.30 sec
log_append_test .................. Passed 16.29 sec
log_cas_test ..................... Passed 43.20 sec

[sherman-the-tank]

Yeah, let's tackle this 👍