[feat] add JUnit Pioneer to dev dependencies

[aepfli]

While developing #810, we saw the need for overwriting environment variables during testing. As this is not a trivial task to do, we looked for easy-to-use alternatives in the JUnit world and 'stumbled' upon JUnit Pioneer.

JUnit Pioneer offers a great set of additional utilities for JUnit tests, like setting env variables, setting system properties, default locales, etc. This could be a great extension of our testing capabilities without adding another dependency for our provided libraries.o

wdyt?

[Kavindu-Dodan]

+1 from my end as this enables provider authors to test the behavior based on environment variables.