Bug 17633: Do not call set_preference from tests

Bug 17633: Do not call set_preference from tests

If memcached is running, the pref's value will be set in cache, what we
do not want.

t/db_dependent/Context.t and t/db_dependent/sysprefs.t are exceptions
for obvious reasons.

Test plan:
Make sure all these tests still pass

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>