CI: Add MySQL e2e tests

[michaelbromley]

We have e2e tests in CI using SQLite, Postgres and MariaDB. However, we need to add MySQL. Even though MariaDB is supposed to be a drop-in replacement, there are some differences which lead to issues like #331

[michaelbromley]

This is currently proving problematic due to bugs in TypeORM's schema sync code. With MySQL, TypeORM attempts to re-create certain foreign key constraints every time, resulting in:

error 5/7/20, 14:14 - [TypeORM] Query error: Error: ER_FK_DUP_NAME: Duplicate foreign key
   constraint name 'FK_0e6f516053cf982b537836e21cf', "ALTER TABLE `product_variant` ADD
   CONSTRAINT `FK_0e6f516053cf982b537836e21cf` FOREIGN KEY (`featuredAssetId`) REFERENCES
   `asset`(`id`) ON DELETE NO ACTION ON UPDATE NO ACTION" -- [undefined] 

This means that the e2e tests cannot run. There are numerous issues on the TypeORM repo relating to schema sync.

It is something Vendure users also run into on doing routine schema migrations - lots of unnecessary migration code. I am tempted to dive in and try to fix it myself upstream...

[michaelbromley]

See #333 for a deeper analysis of the problems here.

[michaelbromley]

Got this working at last with 136b3e9! And it was good to put in the effort as I found a couple of compatibility issues where MySQL does not work the same as MariaDB that needed fixing.