[FSSDK-11236] refactor decision service tests #1069

Job	Run time
lint	1m 10s
unit_tests (16)	1m 57s
crossbrowser_and_umd_unit_tests	6h 0m 13s
unit_tests (18)	1m 45s
unit_tests (20)	1m 36s
unit_tests (22)	1m 35s
fullstack_production_suite / test	4s
integration_tests / test	4s
lint_markdown_files / lint	50s
Coveralls coverage	2s
	6h 9m 16s

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