Write stream implementation (#19)

Defines a WriteStream structure for buffering writes, similar to the existing ReadStream. WriteStreams can be created on OutEndpoints by using ep.NewStream().

fakelibusb_test.go

@@ -192,8 +192,16 @@ func (f *fakeLibusb) setIsoPacketLengths(*libusbTransfer, uint32) {}
 
 // waitForSubmitted can be used by tests to define custom behavior of the transfers submitted on the USB bus.
 // TODO(sebek): add fields in fakeTransfer to differentiate between different devices/endpoints used concurrently.
-func (f *fakeLibusb) waitForSubmitted() *fakeTransfer {
-	return <-f.submitted
+func (f *fakeLibusb) waitForSubmitted(done <-chan struct{}) *fakeTransfer {
+	select {
+	case t, ok := <-f.submitted:
+		if !ok {
+			return nil
+		}
+		return t
+	case <-done:
+		return nil
+	}
 }
 
 // empty can be used to confirm that all transfers were cleaned up.