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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2