tests/narrow-library.sh

requirements: also add a fncache constant

Continue the cleanup to the remaining requirements

Differential Revision: https://phab.mercurial-scm.org/D10109

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