[FSSDK-12097] refactor OptimizelyUserContext tests using TS and vitest #1439

Job	Run time
lint	1m 39s
unit_tests (20)	1m 47s
unit_tests (22)	1m 43s
unit_tests (24)	1m 35s
integration_tests / test	7s
unit_tests (18)	1m 50s
fullstack_production_suite / test	6s
lint_markdown_files / lint	1m 16s
Coveralls coverage	5s
	10m 8s

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