tests/narrow-library.sh

hg-core: return `Err` on decompression error (D8958#inline-15004 followup)

Differential Revision: https://phab.mercurial-scm.org/D9100

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