test: require host-environment in system-defaults tests

system-defaults reads `env-bsd-p` (host-environment) and `user-home-dir` (user-constants) at load, but the module declares both only via eval-when-compile. Loading the compiled module in isolation leaves `env-bsd-p` void, so the test failed whenever it ran outside a full init. I added the host-environment require alongside the existing user-constants require so the unit loads standalone. The production fix (promoting those eval-when-compile requires to a runtime require) is Phase 2 work, recorded in the module inventory.

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