Add test for grep() and wantarray

Add test for grep() and wantarray

Subject: Re: subscript when wantarray

In <199702172207.RAA20755@rio.atlantic.net>, Chip Salzenberg writes:
:First, here's a patch that makes the first case print "2"
[...]
:Second, the reason that it prints "2" instead of "3" is that
:C<(undef)[0]> evaluates to the empty list in an array context,
:but to C<undef> in a scalar context.  This is on purpose.
Yep, I expected that.

Patch worked fine here; below is an extra test using the same code as my
original example.

p5p-msgid: <199702181105.LAA17895@tyree.iii.co.uk>