Add testing to detect errors of omission in "pin" dependency creation.

It's essential that initdb.c's setup_depend() scan each system catalog
that could contain objects that need to have "p" (pin) entries in pg_depend
or pg_shdepend.  Forgetting to add that, either when a catalog is first
invented or when it first acquires DATA() entries, is an obvious bug
hazard.  We can detect such omissions at reasonable cost by probing every
OID-containing system catalog to see whether the lowest-numbered OID in it
is pinned.  If so, the catalog must have been properly accounted for in
setup_depend().  If the lowest OID is above FirstNormalObjectId then the
catalog must have been empty at the end of initdb, so it doesn't matter.
There are a small number of catalogs whose first entry is made later in
initdb than setup_depend(), resulting in nonempty expected output of the
test, but these can be manually inspected to see that they are OK.  Any
future mistake of this ilk will manifest as a new entry in the test's
output.

Since pg_conversion is already in the test's output, add it to the set of
catalogs scanned by setup_depend().  That has no effect today (hence, no
catversion bump here) but it will protect us if we ever do add pin-worthy
conversions.

This test is very much like the catalog sanity checks embodied in
opr_sanity.sql and type_sanity.sql, but testing pg_depend doesn't seem to
fit naturally into either of those scripts' charters.  Hence, invent a new
test script misc_sanity.sql, which can be a home for this as well as tests
on any other catalogs we might want in future.

Discussion: https://postgr.es/m/8068.1498155068@sss.pgh.pa.us