Patch Subset Server Conformance Tests Finished

Some exciting news, last week I finished implementing the conformance tests
for all server side conformance statements in the spec related to the patch
subset protocol. The tests can be found here:
https://github.com/w3c/ift-server-tests

Additionally, as I've developed the tests I've been updating/fixing the
patch subset server implementation so that it passes all of the tests. So
you can currently run the full test suite against the demo incremental
transfer server we have running at
https://fonts.gstatic.com/experimental/patch_subset/demo.html and all the
tests pass.

*In other words, we now have a full server conformance test suite and a
fully compliant server implementation!*

Example command to run the tests:

python3 test_server.py "fonts.gstatic.com"
"/experimental/patch_subset/ofl/montserrat/Montserrat[wght].ttf" "<path to
local copy of Montserrat[wght].ttf (from https://github.com/google/fonts)>"

Next steps will be to develop Range Request conformance tests, which should
be fairly simple. I believe these will mostly end up being tests for
correctly handling of the http range request headers. Then lastly, the
client conformance tests for both patch subset and range request.