Fix flaky TestJDBCQueries.

The class used to set up a database and populate the tables for all
tests. However, some tests do insert new tuples to the tables, and other tests
assert on the tuple count based on the initialized values. When junit runs the
tests in a different order, which can happen sometimes, tests fail.

I added two methods to populate and clear all tables before and after each test
so that they don't have to depend on each other.