Correct volatility markings of a few json functions.

json_agg and json_object_agg and their associated transition functions
should have been marked as stable rather than immutable, as they call IO
functions indirectly. Changing this probably isn't going to make much
difference, as you can't use an aggregate function in an index
expression, but we should be correct nevertheless.

json_object, on the other hand, should be marked immutable rather than
stable, as it does not call IO functions.

As discussed on -hackers, this change is being made without bumping the
catalog version, as we don't want to do that at this stage of the  cycle,
and  the changes are very unlikely to affect anyone.
pg_test_fsync: Update output format

Apparently, computers are now a bit faster than when this was first
added, so we need to make room for a digit or two in the ops/sec format.

While we're at it, adjust some of the other output for a more consistent
line length.