Clean up test fixtures: remove dev partitions and consolidate

Summary

• Remove nixica-dev.nix from all test fixtures — tests don't need a dev partition
• Delete it-evaluates-properly/ — redundant (covered by other tests)
• Consolidate it-can-be-imported-twice/ from 5 files into single configuration.nix
• Enable child-pipelines.configless = true in nixica's own CI so test flakes are CI'd without needing .#gitlab-ci

Result: 3 tests, each with flake.nix + nix/configuration.nix. No dev partition boilerplate.

All test flakes verified to evaluate successfully with nix flake show.

Depends on: gitlab-ci!66 (configless child pipeline mode)

Closes #7