Do not assume interface numbers follow the slice indices.
This is a continuation of
9ad54830f4
which tried to solve the problem of non-contiguous interface indices;
this commit modifies another code path that had the same assumption.
* Add an invalid interface descriptor to one of the test devices.
* Add a test for getting that interface description.
* Handle the malformed interface/alternate setting descriptors, where
interface/alt numbers don't follow the spec (not 0-based, not contiguous
etc.)
Addresses issue #65.
* Store a reference to libusb implementation in the context, transfers
and some more places. Remove the global libusb variable.
* Parallelize tests.
* Fix the link in README.
* 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.
