fix: add automatically generated testindex to gitignore #5995

Job	Run time
sample_operators_tests (sample-operators/tomcat-operator)	2m 41s
sample_operators_tests (sample-operators/webpage)	3m 5s
sample_operators_tests (sample-operators/leader-election)	3m 37s
sample_operators_tests (sample-operators/mysql-schema)	2m 51s
	12m 14s

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