tests/narrow-library.sh

vfs: add NotImplementedError version of join

Again, this helps out pytype.

Differential Revision: https://phab.mercurial-scm.org/D7266

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