large object regression tests, take two

This is the latest version of the large object regression test I have been
working on. Note that a prerequisite for this version of the test is the
patch I made to psql to make it not output on \lo_* commands in quiet mode
is required (also attached, it's small).

Sorry that it still makes use of the sed trickery like the copy test does,
but:

1) I think the server side lo_(import|export) functions need to be tested
as well as the psql variants

2) ISTM that making assumptions about the working directory of psql
during
the regression tests would open a can of worms, especially wrt VPATH
builds where the data files could be in a completely separate tree from
the regression tests.

Thoughts?

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