Fixed MDEV-4033 Unable to use slave's temporary directory /tmp - Can't create/write to file '/tmp/SQL_LOAD-' (Errcode: 17 "File exists")
- Cache value of check_temp_dir() to avoid race condition
- Set $rpl_server_count to avoid error in show_rpl_debug_info.inc

mysql-test/suite/multi_source/info_logs.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/multisource.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/relaylog_events.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/reset_slave.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/simple.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/skip_counter.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/status_vars.test:
  Set $rpl_server_count
sql/slave.cc:
  Cache value of check_temp_dir() to avoid race condition