tests: replace some `python` by `"$PYTHON"` to not only use Python 2
I deleted my `python` binary as a test and a few tests started
failing.
Differential Revision: https://phab.mercurial-scm.org/D10250
#!/bin/sh
HGENCODING=UTF-8
export HGENCODING
echo ".. -*- coding: utf-8 -*-" > gendoc.txt
echo "" >> gendoc.txt
LANGUAGE=$1 "$PYTHON" "$TESTDIR/../doc/gendoc.py" >> gendoc.txt 2> /dev/null || exit
echo "checking for parse errors"
"$PYTHON" "$TESTDIR/../doc/docchecker" gendoc.txt
"$PYTHON" "$TESTDIR/../doc/runrst" html gendoc.txt /dev/null