test: Add test coverage for log custom event #718

Job	Run time
native-unit-tests (iOS, mParticle-Apple-SDK)	7m 34s
native-unit-tests (tvOS, mParticle-Apple-SDK-NoLocation)	4m 0s
native-unit-tests (iOS, mParticle-Apple-SDK-NoLocation)	5m 47s
native-unit-tests (tvOS, mParticle-Apple-SDK)	4m 5s
	21m 26s

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