Print the actual value of each function result that's expected to be exact,
rather than merely emitting a NULL if it's not right.  Although we print
these with extra_float_digits = 3, we should not trust that the platform
will produce a result visibly different from the expected value if it's off
only in the last place; hence, also include comparisons against the exact
values as before.  This is a bit bulkier and uglier than the previous
printout, but it will provide more information and be easier to interpret
if there's a test failure.

Discussion: <18241.1461073100@sss.pgh.pa.us>