Fix unit-test code JdbcDataSourceTest.java

This JDBC unit-test (java/ext-data-source/jdbc/src/test/java/org/apache/impala/extdatasource/jdbc/JdbcDataSourceTest.java) was implemented with H2 database. We don't have H2 in our environment and the code was out of date. We need to rewrite this unit-test in Postgres.