(Blind) fix Perl splitting of strings at newlines

I forgot that Windows represents newlines as \r\n, so splitting a string
at /\s/ creates additional empty strings.  Let's rewrite that as /\s+/
to see if that avoids those.  (There's precedent for using that pattern
on Windows in other scripts.)

Previously: 91bdf499, 8ed428dc, 650b9670.

Per buildfarm, via Tom Lane.
Discussion: https://postgr.es/m/3144460.1615860259@sss.pgh.pa.us

src/test/modules/libpq_pipeline/t/001_libpq_pipeline.pl

@@ -16,7 +16,7 @@ $ENV{PATH} = "$ENV{PATH}:" . getcwd();
 
 my ($out, $err) = run_command(['libpq_pipeline', 'tests']);
 die "oops: $err" unless $err eq '';
-my @tests = split(/\s/, $out);
+my @tests = split(/\s+/, $out);
 
 for my $testname (@tests)
 {