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>

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With this PR, all tests except 1 pass. See https://github.com/phpmyadmin/phpmyadmin/pull/1056