I don't see one, but I think there ought to be one.
My revised libpq managed to pass the regression tests
even though PQfn() was broken; I didn't discover that
until I tried to run the large-object example in
src/test/example/testlo.c. (Perhaps that file could
be converted into a regression test with little effort?)
BTW, src/test/example/testlo2.c looks like a waste of
disk space. It seems to be an old version of testlo.c.
regards, tom lane