Move slow tests in HHVM to group medium

These tests were failing on HHVM because of the default 3 second timeout.
Yes, HHVM is supposed to be faster, but there's a "warmup" time for scripts
in JIT mode (which is on by default). I suspect that's why they're failing.

Signed-off-by: Jeff Chan <jefftchan@gmail.com>
Merge pull request #1055 from jeffchan/up-test-time-limit

Move slow tests in HHVM to group medium