test: reset ExternalIDGenServiceMock between tests to fix flaky external-state ITs #7153

Job	Run time
sample_operators_tests (sample-operators/mysql-schema)	2m 45s
sample_operators_tests (sample-operators/operations)	8m 1s
sample_operators_tests (sample-operators/tomcat-operator)	2m 35s
sample_operators_tests (sample-operators/webpage)	3m 0s
sample_operators_tests (sample-operators/leader-election)	3m 19s
	19m 40s

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