Convert the middle test loops of closure.t to test.pl

Convert the middle test loops of closure.t to test.pl

The nested loops build tap-generating test programs, spawn them, capture their
output, directly print the output, and also run a rudimentary pass of it to
look for /not ok/, or anything on STDERR. Retain the same structure, and retain
the existing spawning code which (a) works and (b) has comments about being
careful to avoid problems with redirection and inherited STD*, but switch to
using test.pl in the test programs, giving each test an identifying
description, and better diagnostics if anything fails.