Fix use of loop variable

I was having this test fail when malware scanning is enabled with:
Use of uninitialized value $_ in concatenation (.) or string at t/web/charting.t line 14.

In perlsyn under https://perldoc.perl.org/perlsyn#Foreach-Loops it says:

The foreach keyword is actually a synonym for the for keyword, so you can use
either. If VAR is omitted, $_ is set to each value.

VAR is set (it is $n), so perhaps this usage of $_ while VAR is set
is undefined.

Author: puck

t/web/charting.t

@@ -9,7 +9,7 @@ for my $n (1..7) {
     my $ticket = RT::Ticket->new( RT->SystemUser );
     my $req = 'root' . ($n % 2) . '@localhost';
     my ( $ret, $msg ) = $ticket->Create(
-        Subject   => "base ticket $_",
+        Subject   => "base ticket $n",
         Queue     => "General",
         Owner     => "root",
         Requestor => $req,
@@ -19,8 +19,8 @@ for my $n (1..7) {
         MIMEObj   => MIME::Entity->build(
             From    => $req,
             To      => 'rt@localhost',
-            Subject => "base ticket $_",
-            Data    => "Content $_",
+            Subject => "base ticket $n",
+            Data    => "Content $n",
         ),
     );
     ok( $ret, "ticket $n created: $msg" );