scripts/t: Do not use system() string form

This passes the arguments as a list, and stops ignoring stderr.

scripts/t/Dpkg_Shlibs.t

@@ -180,7 +180,7 @@ sub save_load_test {
 
     is_deeply($dup, $symfile, $comment);
     if (-f $symfile->{file}) {
-	is( system(sprintf("diff -u '%s' '%s' >&2", $symfile->{file}, $save_file->filename)), 0,
+	is(system('diff', '-u', $symfile->{file}, $save_file->filename), 0,
 	    basename($symfile->{file}) . ' dumped identical');
     }
 }

scripts/t/merge_changelogs.t

@@ -36,7 +36,7 @@ sub test_merge {
 	  wait_child => 1, nocheck => 1);
     my $res = compare($expected_file, $fh->filename);
     if ($res) {
-	system("diff -u $expected_file $fh->filename >&2");
+	system('diff', '-u', $expected_file, $fh->filename);
     }
     ok($res == 0, "merged changelog matches expected one ($expected_file)");
 }