tests: remove sys.executable from "required tools"
In practice this doesn't appear to have been true for some time - we
reference Python using the $PYTHON variable in all the tests now
(which we have to for PyPy and Python 3), and I've been using
~/.../python.exe to test with tip of the cpython 3.6 release branch
while working on manifest tests in Python 3 and everything seems to be
just fine. The only real observable difference from this change is
that I stop getting a warning about python.exe not being a thing on
$PATH, which seems like an improvement.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.