Merge large_object.sql test into largeobject.source.

Merge large_object.sql test into largeobject.source.

It seems pretty confusing to have tests named both largeobject and
large_object.  The latter is of very recent vintage (commit ff992c074),
so get rid of it in favor of merging into the former.

Also, enable the LO comment test that was added by commit 70ad7ed4e,
since the later commit added the then-missing pg_upgrade functionality.
The large_object.sql test case is almost completely redundant with that,
but not quite: it seems like creating a user-defined LO with an OID in
the system range might be an interesting case for pg_upgrade, so let's
keep it.

Like the earlier patch, back-patch to all supported branches.

Discussion: https://postgr.es/m/18665.1500306372@sss.pgh.pa.us