feat: Migrate java-test-helpers to java-core. #134

Job	Run time
test-packaging	1m 22s
test-platforms-and-versions (ubuntu-latest, 19)	1m 36s
test-platforms-and-versions (ubuntu-latest, 8)	1m 37s
test-platforms-and-versions (ubuntu-latest, 17)	1m 28s
test-contract-tests	1m 32s
test-platforms-and-versions (ubuntu-latest, 11)	1m 38s
	9m 13s

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