Rewrite test generators and update exercises

[grzegorz-bielski]

In the scope of Scala 3 migration in #836 old test generators were removed as they were quite stale and didn't work as expected for some time now, after upgrading the libs during migration (mainly play-json) they stopped working completely.

#836 introduced a new POC approach to test generators based on the old one in the testgen module. It's not yet used.

For each exercise, we need to:

• make a new test gen class
• run the test gen and include it in the exercise folder
• fix any errors and warnings in the tests or examples

In addition the core test gen code from POC could use following improvements:

• template with more idiomatic Scala 3 test suite syntax
• removal of the Twirl (?)
• tighter integration with the config.json

This is blocked by: #842