update EventBaseTest to use precise timing with libevent2

Summary:
Change the EventBaseTest code to request precise timing from libevent.
libevent 2.1+ uses a coarse monotonic clock by default on Linux.
This causes several of folly's timing-related tests for EventBase to fail most
of the time.

This changes the tests to request precise timing from libevent2, using the
`EVENT_PRECISE_TIMER`.  While we can request this programmatically, doing so
portably across libevent versions is awkward, so the environment variable is
the easiest solution for now.

Alternatively we could change the tests to use larger, coarser timeouts to
make them pass reliably.  However, this would make the tests slower.

Differential Revision: D15528322

fbshipit-source-id: 20dc55de36235e63910cf7fb5b67c6f2d509cc15