Change Elastic Agent output mode to latency for system tests

[ruflin]

Running some system tests, it seems the system is waiting on events for quite some time. Looking at the defaults set for Elastic Agent, the default is "Balanced". For the test suite, having quick test runs is most important especially as the amount of data is low. I'm proposing to change the Elastic Agent for elastic-package to latency: https://www.elastic.co/guide/en/fleet/8.14/es-output-settings.html#es-output-settings-performance-tuning-settings