Postgres tests should not fail if a Postgres server is not available
Postgres tests should not fail if a Postgres server is not available

The MySQL tests pass if a MySQL server is not installed, and the Postgres
tests should behave similarly. Ideally, both sets of tests would mark
themselves as skipped using `t.Skip()`, but that would require more
extensive changes.

Martin2112/trillian,RJPercival/trillian,daviddrysdale/trillian,gdbelvin/trillian,Martin2112/trillian,daviddrysdale/trillian,google/trillian,gdbelvin/trillian,google/trillian,Martin2112/trillian,AlCutter/trillian,gdbelvin/trillian,AlCutter/trillian,daviddrysdale/trillian,RJPercival/trillian