MDEV-27775 : Some Galera tests fail on FreeBSD due to "unknown signal 9"

Replace 9 with KILL

mysql-test/suite/galera/include/kill_galera.inc

 --echo Killing server ...
 
+if (!$kill_signal)
+{
+--let $kill_signal = KILL
+}
+
 # Write file to make mysql-test-run.pl expect the crash, but don't start it
 --let $_server_id= `SELECT @@server_id`
 --let $_expect_file_name= $MYSQLTEST_VARDIR/tmp/mysqld.$_server_id.expect
@@ -7,13 +12,15 @@
 
 # Kill the connected server
 --disable_reconnect
+--let KILL_SIGNAL_VALUE = $kill_signal
 --let KILL_NODE_PIDFILE = `SELECT @@pid_file`
 
 --perl
+        my $kill_sig = $ENV{'KILL_SIGNAL_VALUE'}
         my $pid_filename = $ENV{'KILL_NODE_PIDFILE'};
         my $mysqld_pid = `cat $pid_filename`;
         chomp($mysqld_pid);
-        system("kill -9 $mysqld_pid");
+        system("kill -s $kill_sig $mysqld_pid");
         exit(0);
 EOF