Lower source-postgres CAT duration

[alafanechere]

As source-postgres now passes CAT we can work on making it run faster.
We don't want this test suite to be the bottleneck of the source-postgres tests.

Original cat duration: 7mn46

[alafanechere]

First optimization:

When running CAT against a CDC config the initial_waiting_time setting has a huge impact on the CAT duration. This setting determines the time for which debezium will wait for new records. As we're not inserting records to postgres in CAT this duration can be lowered to a minimum.

Original value: 300s (the default)
New value: 1s

To set this value to 1 we have to set the is_test setting to True, otherwise the config validation fails as normally the lowest acceptable valust is 120s

Speed gain

07mn41s - 03mn54s = 3mn47s