mysql-test/mysql-test-run.pl:
  Move some opt_extern hacks to same place.
  Remove duplicate opt_skip_im=1 if opt_extern
mysql-test/t/fix_priv_tables.test:
  Skip test if $MYSQL_FIX_PRIVILEGE_TABLES is not set
mysql-test/t/system_mysql_db_fix30020.test:
  Skip test if $MYSQL_FIX_SYSTEM_TABLES is not set
mysql-test/t/system_mysql_db_fix40123.test:
  Skip test if $MYSQL_FIX_PRIVILEGE_TABLES is not set