test-wrap-argv: use map and join instead of a for cycle

test-wrap-argv: use map and join instead of a for cycle

We have a list of parameters in @args, that need to be rewrapped
and separated by a space and escaped newline: " \\\n", with the
exception of the last one, which only needs a newline.

Instead of a for cycle, rewrap the individual arguments using map,
and interleave them with escaped newlines by using join.