zstd: hack include order to ensure that our zstd.h is found
If the regular Python CFLAGS include directories that already have the
zstd headers available, a different and possible incompatible version
can be picked up otherwise. Sadly, it seems like Python has no easy way
to prefix flags before the rest.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.