XS-APItest/t/call.t: make loops more flexible

XS-APItest/t/call.t: make loops more flexible

There's a loop which tests eval_pv, eval_sv, call_sv with various
types of argument. Currently the argument is determined by an integer
in a loop (0..3) with various values derived on an ad-hoc basis
from that index. Instead put all the data into an array of arrays
and iterate over that instead.

Similarly for the function names (eval_pv et al), loop over the names
rather than over 0..2.

This should make no functional change to what is tested, but makes the
test code clearer and more expandable.