Testing: Add 'log.format=json' condition to test matrix

https://github.com/elastic/logstash/issues/6906 is caused by an exception thrown by a RabbitMQ object while doing JSON serialization for a logger call.

Making all our tests run with 'log.format' set to json will hopefully help catch things like this.

Start by locating the test matrix and existing handling of the log.format setting, then review the related issue 6906 for the failure context. Run the current test suite with log.format=json where supported; done means the matrix exercises JSON logging and the tests pass without serialization exceptions.

Written by the indexing model from the issue text.