Add missing tests for ML APIs (search and delete) #1566

Job	Run time
test-opensearch-spec (version=1.3.17, hub=opensearchproject, tests=default)	1m 20s
test-opensearch-spec (version=2.0.0, hub=opensearchproject, tests=default)	1m 13s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=default)	1m 35s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/index_state_management)	52s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/ml)	1m 27s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=routing)	1m 14s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=snapshot)	55s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/streaming)	58s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/notifications)	51s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/query_insights)	51s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/workload-management)	52s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/analysis)	1m 5s
test-opensearch-spec (version=2.19.0, hub=opensearchstaging, tests=default)	1m 41s
test-opensearch-spec (version=3.0.0, hub=opensearchstaging, tests=default)	1m 34s
merge-coverage	0s
	16m 28s

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