add test which checks that all strings are defined as constants + verify if all constants are still in use

[trz42]

One variant could be to import modules that define string constants and then scan python scripts if any of those string constants' values are used.

Another variant could be to scan all python scripts and check if any strings are used instead of string constants.

The latter variant would be better to ensure that no new strings are introduced.

A third check could verify if all defined string constants are still in use.