Fixes autotest downgrade failures for 8.0 due to refused TCP connection.

In 8.0 the mysqld has been configured with "skip-grant-tables" which enables another
server option "skip-networking" causing mysqld server not to listen any TCP connections.

Hence this patch modifies the configuration file by removing "skip-grant-tables".

Additionally, changed tables engine from "myisam" to "innodb" to cope with the default charset
changes causing error "1071:Specified key was too long; max key length is 1000 bytes.".
 From 5.7 to 8.0 charset was changed from "latin1" to "utf8mb4". This means that a varchar(255)
key that took 255 * 1 = 255 bytes in MySQL 5.7 now takes 255 * 4 = 1020 bytes (larger than myisam
limit of 1000 bytes) in MySQL 8.0.