move all tests to TestItems.jl and TestItemsRunner.jl

[CarloLucibello]

This would give us better vscode integration and is more like to have long-term support compared to e.g. ReTestItems.jl.

As a part of this, should also document in the developer docs how to conveniently tests using TestEnv.jl.

First step for addressing this issue is to move to TestItems the GNNLux tests.