Enable JIT for unit tests in HHVM

By default HHVM does not enable the JIT for scripts run from the
command line as they are expected to run for fractions of a second.
These unit tests run long enough that enabling the JIT should be
a net win.  Worth trying at least.

Bug: 62278
Change-Id: Ie5f6ef8952451e742f84004c2312e66dba2acd7e