Polyhedron._test_product, _test_dilation: Skip tests if test prereqs cannot be imported

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CC:  @kliem

Component: **refactoring**

Author: **Matthias Koeppe**

Branch/Commit: **[`97e6647`](https://github.com/sagemath/sagetrac-mirror/commit/97e664718f95409177880080c2b763de64775ed1)**

Reviewer: **Jonathan Kliem**

_Issue created by migration from https://trac.sagemath.org/ticket/33403_

