Move test environment to [extras]

[Da-Be-Ru]

As mentioned in #185 and elsewhere, it seems best to move the test environment from its own Project.toml into the [extras] section.

However, the Aqua compat tests will now fail because I didn't define any [compats] for these new [extras]. I was not entirely sure which versions I should include in there for the sake of backward compatibility. What would be an appropriate choice here?

EDIT: Btw, we overlooked the [compat] additions in the corresponding PR to ExtendableSparse because there are no Aqua tests running in there.

[j-fu]

Compat entries of packages in [extras] are not relevant if the package is used as a dependency, they are used only during testing. So there are essentially no backward compatibility issues, and we can use the newest possible version.
Please note that for VoronoiFVM, some of the standard Aqua tests (partially) fail, and fixing these seems to require breaking changes, see #187.

[Da-Be-Ru]

I adjusted the [compat]s so they should hopefully run through the CI-tested julia versions 1.10.9 and 1.11.4.
But I also noticed that the julia compat entry is set to 1.9 despite it not being tested by any pipeline. Do we keep that up or should we bring these versions in line? Perhaps this is a question for a different issue or PR.