t: adapt test_copy_bytes() to not use Perl

The `test_copy_bytes()` helper function copies up to N bytes from stdin
to stdout. This is implemented using Perl, but it can be trivially
adapted to instead use dd(1).

Refactor the helper accordingly, which allows a bunch of tests to pass
when Perl is not available.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

t/test-lib-functions.sh

@@ -1640,17 +1640,7 @@ test_match_signal () {
 
 # Read up to "$1" bytes (or to EOF) from stdin and write them to stdout.
 test_copy_bytes () {
-	perl -e '
-		my $len = $ARGV[1];
-		while ($len > 0) {
-			my $s;
-			my $nread = sysread(STDIN, $s, $len);
-			die "cannot read: $!" unless defined($nread);
-			last unless $nread;
-			print $s;
-			$len -= $nread;
-		}
-	' - "$1"
+	dd ibs=1 count="$1" 2>/dev/null
 }
 
 # run "$@" inside a non-git directory