KAFKA-19418: Fix for flaky verifiable producer system test

[lianetm]

Fix to avoid flakiness in verifiable producer system test. The test lists running processes and greps to find the VerifiableProducer one, but wasn't providing an specific pattern to grep (so flaky if there were more than one process containing the default grep pattern "kafka")

Fix by passing a "proc_grep_string" to filter when looking for the VerifiableProducer process.

All test pass successfully after the change.

[lianetm]

Here's the system test run for this change, all tests pass https://confluent-open-source-kafka-branch-builder-system-test-results.s3-us-west-2.amazonaws.com/trunk/2025-06-18--001.dca32b28-bde7-417a-b440-06d06e2794bf--1750278686--lianetm--lm-verifiable-producer-fix--332b5ebc78/report.html