Add variables for house inputs

[BNAndras]

http://forum.exercism.org/t/well-defined-input-and-output-requirements-in-exercises/17809/10

No need to rerun tests

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

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[glennj]

Not sure why that check is failing: dart run bin/check_formatting.dart

[BNAndras]

Yeah, I have sporadic fights with the formatter because the CI wants line lengths of 120, but the default for dart format is 80. The formatter complains if a line is formatted to be shorter when it originally would have been under the line length threshold. I don't recall running dart format so maybe a plugin ran it for me and I didn't catch that it happened.