Show x,y diff holistically in test() failure

[MichaelChirico]

Also not 100% satisfied with the index representation. Maybe something like

First 6 of 980 differing elements (1000 total):
 index  x  y
    11 11 20
    12 12 19
    13 13 18
    14 14 17
    15 15 16
    16 16 15
Types: x integer, y integer

Originally posted by @ben-schwen in #7836 (comment)

In #7836, we eschewed the above representation of a test() failure because it requires a deeper reconsideration of the code structure, which currently shows the failed objects one-by-one:

failPrint(x, ...)
failPrint(y, ...)

Doing the above would entail something more like:

failPrint(x, y, ...)