Tests assume too much about Time::HiRes internals

[dur-randir]

Starting with Time::HiRes version 1.9754, test t/unit_stats.t from this module started to fail. This is because it redefines gettimeofday() but not tv_interval() and assumes that the latter will call the former. But this is no longer the case due to optimizations in Time::HiRes.

The original discussion can be found here https://rt.cpan.org/Ticket/Display.html?id=124450.