Some tests may be sensitive to system time, and fail intermittently when the clock ticks over in the middle of a test. Similarly, application logic may sometimes fail if the clock ticks over unexpectedly. To expose such code, we should use a mock clock during testing that ticks up one second between every call.
The mock clock should at least be used by the wfTimestamp* functions and the MWTimestamp and ConmvertibleTimestamp classes. It does not need to, and maybe should not, affect calls to PHP's built in time() function.