Add integration tests with testcontainers

[kevindeyne]

Testcontainers should be set up so that we have actual connections with test databases and can test model creation and data generation. Mostly my interest right now is in providing a good regression base, since I don't always have time to set up databases manually to do in-depth testing.

The following should be used:

• MariaDB (https://www.testcontainers.org/modules/databases/mariadb/)
• MySQL (https://www.testcontainers.org/modules/databases/mysql/)
• Postgresql (https://www.testcontainers.org/modules/databases/postgres/)

[kevindeyne]

Added basic (1 table, id + string value) build/generate test for all 3 dbs.

Should expand these tests to also test more complex generation (a significant amount of datatypes should be present, different distributions of data should be tested, some relations, some indexes)

[kevindeyne]

Look into other supported DB as well:

• https://github.com/FirebirdSQL/firebird-testcontainers-java

Might want to remove the in-memory DBs I have listed as 'supported' since those don't really make sense in our context.