Config and interface description (#16)

* Add APIs for config and interface descriptors. Split out the common
parts of selecting a config descriptor from device desc and
selecting a setting descriptor from a config desc.

* Parallelize the few tests that actually can be parallelized safely.
Add comments where they can't. Note to self: it would be beneficial
to restructure the fakelibusb to index all properties of the lib
with the used context. That way a libusb implementation wouldn't need
to be referred via a shared variable.

transfer_stream_test.go

@@ -106,6 +106,7 @@ func (r readRes) String() string {
 }
 
 func TestTransferReadStream(t *testing.T) {
+	t.Parallel()
 	for tcNum, tc := range []struct {
 		desc        string
 		closeBefore int
@@ -211,7 +212,9 @@ func TestTransferReadStream(t *testing.T) {
 			},
 		},
 	} {
+		tcNum, tc := tcNum, tc // t.Parallel will delay the execution of the test, save the iteration values.
 		t.Run(strconv.Itoa(tcNum), func(t *testing.T) {
+			t.Parallel()
 			t.Logf("Case %d: %s", tcNum, tc.desc)
 			ftt := make([]*fakeStreamTransfer, len(tc.transfers))
 			tt := make([]transferIntf, len(tc.transfers))