Changed persistit tests to forkMode=always

This makes their surefire configuration consistent with the rest of
the Titan modules and makes the <argLine>-Xms=... -Xmx=...</argLine>
surefire configuration setting actually have an effect.

<forkMode> is deprecated in newer versions of the surefire plugin, but
we're currently using 2.12.1 everywhere in Titan, and I think a
surefire upgrade would be better handled in a separate commit.