Granular requiredSystemFeatures: build individual attributes in passthru.tests

[Someone]

Currently Ofborg skips all of passthru.tests if at least one of them declares unknown requiredSystemFeatures. Instead Ofborg could filter out the unsupported tests and build the rest. Use-case: this would help avoiding custom attributes like gpuChecks in the following example, https://github.com/NixOS/nixpkgs/blob/24833cd162babe2c4d50b910d439d714fa8897d2/pkgs/development/python-modules/gpuctypes/default.nix#L88-L112