You can define a
nixosConfiguration in your flake file, which has the module you're testing enabled in it.
nixos-container create <name> --flake .#<configuration name> and
nixos-container start <name>
Boom, you can now test the package & module in one flake without polluting your current environment
You can also log in to the containers by doing
nixos-container root-login <name> and have a peak at what's going on