[FSSDK-11236] refactor decision service tests #1072

Job	Run time
crossbrowser_and_umd_unit_tests	6h 0m 13s
lint	1m 15s
unit_tests (16)	2m 10s
unit_tests (18)	1m 42s
unit_tests (20)	1m 39s
unit_tests (22)	1m 37s
fullstack_production_suite / test	6s
integration_tests / test	4s
lint_markdown_files / lint	56s
Coveralls coverage	2s
	6h 9m 44s

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