Make intentional failure unit test more widely compatible

On Mac (maybe elsewhere) this fails with an aggregate error of IPv6 +
IPv4 errors, so the message is empty. Checking the code should be more
compatible everywhere I think.

test/unit-tests.spec.ts

@@ -133,7 +133,7 @@ describe("Usbmux-client unit tests", () => {
 
         const deviceQueryResult = await client.getDevices().catch(e => e);
         expect(deviceQueryResult).to.be.instanceOf(Error);
-        expect(deviceQueryResult.message).to.contain("ECONNREFUSED");
+        expect(deviceQueryResult.code).to.equal("ECONNREFUSED");
 
         await startServer(port);