[BUG] Non-deterministic behavior of mixed cluster YAML tests

[andrross]

OpenSearch provides a mechanism for testing the REST API using a YAML-based DSL. All the YAML tests are run against the code at build time as a part of the :rest-api-spec:yamlRestTest. They are also run as a part of the :qa:mixed-cluster:v2.4.0#mixedClusterTest task via MixedClusterClientYamlTestSuiteIT. When introducing a new feature to the REST API, the proper way to define a new test is to put a skip <version> section in the YAML test definition, which tells the mixed cluster test that this test should not run against an older version which doesn't support the feature being tested. However, if one forgets to add this skip section, the tests may still pass in the mixed cluster tests if the queries happen to hit only the nodes running the new version. See #4852 for a case where this happened. It would be better for the misconfigured test definition to deterministically fail. The obvious approach that stands out to me is to run these YAML tests against an exclusively "old version" cluster to ensure they have the proper guards in place so that any misconfiguration is guaranteed to fail. Are there any other approaches to solve this problem?