tests: don't require kafka client for regular tests (#8662)

## Problem

We're adding more third party dependencies to support more diverse +
realistic test cases in `test_runner/logical_repl`. I ❤️ these
tests, they are a good thing.

The slight glitch is that python packaging is hard, and some third party
python packages have issues. For example the current kafka dependency
doesn't work on latest python. We can mitigate that by only importing
these more specialized dependencies in the tests that use them.

## Summary of changes

- Move the `kafka` import into a test body, so that folks running the
regular `test_runner/regress` tests don't have to have a working kafka
client package.

test_runner/logical_repl/test_debezium.py

@@ -12,7 +12,6 @@ import requests
 from fixtures.log_helper import log
 from fixtures.neon_fixtures import RemotePostgres
 from fixtures.utils import wait_until
-from kafka import KafkaConsumer
 
 
 class DebeziumAPI:
@@ -95,6 +94,8 @@ def debezium(remote_pg: RemotePostgres):
     log.debug("%s %s %s", resp.status_code, resp.ok, resp.text)
     assert resp.status_code == 201
     assert len(dbz.list_connectors()) == 1
+    from kafka import KafkaConsumer
+
     consumer = KafkaConsumer(
         "dbserver1.inventory.customers",
         bootstrap_servers=["kafka:9092"],