tests/narrow-library.sh

run-tests: flush output stream before prompting to accept changes

With py3 on Windows, the prompt to accept changes in an --interactive run wasn't
showing up until after the choice was read from stdin.

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$TESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF