Add test for sprintf ordering by both explicit index and not.

Add test for sprintf ordering by both explicit index and not.

Commit 638ca15 earlier in the 5.23 development cycle corrected a
long-standing bug in sprintf.  Not surprisingly, code outside the
core built on this bug will now exhibit a different behavior.
CPAN library Text-sprintfn is one such case.  One test in its
test suite began to fail; see
https://rt.cpan.org/Ticket/Display.html?id=105989.

This commit adds the test which failed in Text-sprintfn's t/01-basic.t to our
t/op/sprintf.t with the corrected test result.  It also adds a 'printf'
version of that corrected expectation to pod/perlfunc.pod.

For: RT #125956