Add more and useful tests of the p4est library

[sloede]

While we already have 100% test coverage, this is only due to the fact that we do not count the Julia-to-C bindings as code (since it is not officially part of this package). However, it would be great to have some more meaningful tests that exercise some functionality of the p4est library. I am thinking about things like

• Create a non-square mesh
• Refine locally
• Rebalance

This way, we could have higher confidence that the core algorithms of the JLL package and the custom builds that we test are working as they should.

[efaulhaber]

Now that we had two bugs that required workarounds, we should test these features (JuliaPackaging/Yggdrasil#3095 and JuliaPackaging/Yggdrasil#3106) as well.