Support multiple testbeds per module with /modules/<MODULE>/testbeds/<TESTBED>.nix, while validating various invariants.
Link: https://github.com/danth/stylix/pull/758 Reviewed-by: NAHO <90870942+trueNAHO@users.noreply.github.com> Reviewed-by: awwpotato <153149335+awwsomepotato@users.noreply.github.com>