MDEV-18353 fixup. Run mtr test on Windows

mtr supports running processes in background on Windows since 10.8.
Rename shutdown_not_windows.test to shutdown_debug.test, and allow this
test everywhere.

mysql-test/main/shutdown_not_windows.test → mysql-test/main/shutdown_debug.test

-source include/not_windows.inc;
 source include/not_embedded.inc;
 source include/have_debug.inc;
 --echo #
@@ -8,7 +7,7 @@ call mtr.add_suppression('Thread .* did not exit');
 set @old_dbug=@@global.debug_dbug;
 set global debug_dbug='+d,CONNECT_wait';
 select variable_value into @cons from information_schema.global_status where variable_name='connections';
-exec $MYSQL -e 'select sleep(3600)' >/dev/null 2>&1 &;
+exec $MYSQL -e "select sleep(3600)" >/dev/null 2>&1 &;
 let $wait_condition= select variable_value>@cons from information_schema.global_status where variable_name='connections';
 source include/wait_condition.inc;
 source include/restart_mysqld.inc;