WL#4350  - Rearrange spawn worker code
WL#4350
 - Rearrange spawn worker code

mysql-test/mysql-test-run.pl:
   - Rearrange the spawning of workers to make it possible to not
     spawn more workers than number of tests to run
   - This also make it enough to parse args in the main thread

davidl-zend/zenddbi,davidl-zend/zenddbi,natsys/mariadb_10.2,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,ollie314/server,ollie314/server,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,flynn1973/mariadb-aix,ollie314/server,ollie314/server,flynn1973/mariadb-aix,davidl-zend/zenddbi,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,ollie314/server,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,slanterns/server,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server