tests: avoid using rebaseskipobsolete=0
I'm about to delete the `rebaseskipobsolete` config. This patch
updates a test to get hidden commits into the rebase state by using
`hg debugobsolete` instead of setting `rebaseskipobsolete=0`.
Differential Revision: https://phab.mercurial-scm.org/D10270
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=$PYTHON "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF