[Testsuite] Add option to test subset of types

[christiangnrd]

Sorts out supported eltypes into biins and only picks one per bin to run tests on.

Current questions:

• Bins make sense?
• Need to be more reproducible or is it fine to have someone run the full testuite to reproduce?
• If a backend implements it, testsuite won't be able to run on older GPUArrays testsuites

Locally on Metal.jl I went from 8 minutes to 6