Avoid requiring editable builds (`-e`) for running tests

[leofang]

Right now it is quite subtle that the test/benchmark suites are designed to only allow editable builds to run. For normal builds, pytest has a hard time importing the needed modules. I stumbled over this for a few times. https://github.com/NVIDIA/cuda-python/blob/e1e332564c48db556212d59262a149b1a63285e8/docs_src/source/install.md?plain=1#L65 This should be fixed.