Force H2 driver registration to fix SQLException in tests

Running DBTest and SlickPlayIterateesFunctionalTest together
reliably threw a "java.sql.SQLException: No suitable driver found".
This fix forces registration of the H2 driver to make sure a
driver is available.

It shouldn't be necessary to register the driver. Something
is presumably deregistering a driver that it didn't register.
Slick doesn't deregister drivers, so it's probably Play. There
is a recent fix in Play's DB plugin that might resolve this
issue: https://github.com/playframework/playframework/pull/2794.
Once that issue's fixed we can try to remove this workaround
from our tests.
Merge pull request #166 from richdougherty/register-driver-in-tests

Force H2 driver registration to fix SQLException in tests