[Mapping.NonLinear] Add tests for DistanceMultiMapping

[alxbilger]

DistanceMultiMapping was not tested, and it's the first example of a tested non-linear multi mapping.

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[alxbilger]

[ci-build][with-all-tests]

[fredroy]

[ci-build][with-all-tests][force-full-build]