tests: stop running most integration tests with two time zones

[pjonsson]

The uninitialised_postgres_db fixture in the integration tests is parameterized over two time zones. This effectively doubles the number of tests that have to be run, and that also doubles the time it takes to run the tests.

The parameterization was added many years ago when there was most likely not so many tests to run, so doubling the number of tests had less of an impact. We now have more tests, and many of those tests have to be run with both the postgres and postgis driver, and those tests also have to be run with both psycopg2 and psycopg3 for each, so that is 8x as many tests as if there was just one driver/psycopg/time zone to test for.

Some tests should be run with two time zones, but many tests behave exactly the same regardless of time zone, and we should stop testing them with multiple time zones.

[pjonsson]

The linked PR just fixed two tests, so something still needs to be done for most other tests in integration tests, reopening this.