tests/README

match: remove ispartial()

The function was added in d1d69ca78883 (match: add match.ispartial(),
2015-05-15) for use by narrowhg, but narrowhg never ended up needing
it.

To run the tests, do:

cd tests/
python run-tests.py

See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.