Add example unit tests for the JavaScript SDK

[tinahollygb]

Add example unit tests for how to test with the JavaScript SDK.

Possible ideas:

• have the user implement a separate feature repository to fetch features (e.g. featureRepository.getFeatures() and set those in the SDK with setFeatures() or passed into the Context constructor. this allows the user to create a mock of the feature repository's getFeatures() method.

[tinahollygb]

This is somewhat related: growthbook/growthbook#1690