Granular requiredSystemFeatures: build individual attributes in passthru.tests

[SomeoneSerge]

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

[SomeoneSerge]

Apparently this is also an issue for unfree tests: https://logs.ofborg.org/?key=nixos/nixpkgs.256230&attempt_id=15fe1427-f73b-4e06-a084-11cd77b929e4