Fix solution errors and add test to validate solutions

[evrys]

Hallo Alexander!

My friend is a CS student at Hochschule Bonn-Rhein-Sieg, and I've been helping him study for his databases exam using the sql-trainer. However, we've found that some of the tasks can't be answered, often because the provided solution uses invalid SQLite syntax.

This PR adds a jest test which validates the solutionQuery for every task, so it's easy to find which ones are broken. The test structure follows the recommendations for testing given by create-react-app. Tests can be run with npm run test

I've also gone through and fixed 39 queries which were caught by the test. There was however one task (DML 27) where the solutionQuery uses a non-standard Oracle SQL syntax CONNECT BY PRIOR to find all the descendants of a tree structure. I couldn't figure out how to do this in SQLite without an unresonably complex query, so I commented out the task.

Also fixes #2

Thanks for your cool project!

[aahajj]

Hey Evan, I am preparing for the same exam and I found a mistake that is not mentioned in your pull request.
This solutionQuery:

sql-trainer/src/Tasks/DDLTasks.ts

Line 245 in e18dbce

"CREATE VIEW Inspektion AS SELECT am, fahrzeug_id FROM Inspektionen WHERE firma = 'VeServ';",

should be:

      "CREATE VIEW v_inspektion AS SELECT am, fahrzeug_id FROM Inspektionen WHERE firma = 'VeServ';",

Thank you :))

[evrys]

Added to the PR, thanks! Hopefully someone gets time to maintain this project since it's in active use by universities,,