setup mock socket server for testing env

[todo]

ocap-javascript-sdk/packages/ocap-js/test/index.spec.js

Lines 100 to 105 in 068b598

	// TODO: setup mock socket server for testing env
	const subscription = await client.newBlockMined();
	expect(subscription instanceof EventEmitter).toEqual(true);
	expect(subscription.subscriptionId).toBeTruthy();
	expect(Object.keys(client.subscriptions).length).toEqual(1);
	},

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This issue was generated by todo based on a TODO comment in 068b598 when #6 was merged. cc @ArcBlock.