test: reset ExternalIDGenServiceMock between tests to fix flaky external-state ITs #7144

Job	Run time
sample_operators_tests (sample-operators/mysql-schema)	3m 3s
sample_operators_tests (sample-operators/operations)	8m 17s
sample_operators_tests (sample-operators/webpage)	3m 6s
sample_operators_tests (sample-operators/leader-election)	3m 20s
sample_operators_tests (sample-operators/tomcat-operator)	2m 51s
	20m 37s

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