Played around with the tests a bit and fixed some things:

1. Updated the phpunit.xml.dist to get rid of the warning when starting phpunit:

Warning:       Your XML configuration validates against a deprecated schema.
Suggestion:    Migrate your XML configuration using "--migrate-configuration"!

Basically, they changed how code coverage tool configuration is done. New version works (tested it with phpunit/php-code-coverage).

2. When running the test in the mysql group, a migration test failed because it uses the integer display width property (as in INTEGER(2)), which was removed in MySQL 8.0.19 (see here). The test is now skipped on MySQL
3. Updated a MySQL-only test, that used an incorrect query. The query used the table name to access a column in a subquery, instead of the subquery alias. Hard to say why this fails now/on MySQL 8, since it is a semantic problem unrelated to the DBMS. It might have worked before Bugfix/column names from subquery #1714 (even though the test did not fail there). Anyway, I changed the alias, now it works.