KAFKA-20502: Add IQv2 isolation-level integration tests (#23077)

KIP-892 adds interactive-query isolation levels. The support spans many
store implementations and, for IQv2, all query runners. Until now, only
unit tests covered this behavior.

This change adds the IQv2 counterpart to the IQv1 isolation-level
integration tests from KAFKA-20501. It proves that a READ_COMMITTED
query sees only committed records, and a READ_UNCOMMITTED query sees
uncommitted writes, across all store shapes and query runners. The tests
reuse the shared IsolationLevelIntegrationFixtures harness added by
KAFKA-20501.

Reviewers: Bill Bejeck <bbejeck@apache.org>

Author: nicktelford