add test for scotch

Simon Praetoriusrequested to merge
feature/ptscotch_test into master

Summary

This MR adds a test scotchtest.cc to check that scotch is found and works correctly. The actual scotch test just runs a few standard functions to create a graph and check it validity. The test is added to test the cmake find-package FindPTScotch.cmake and its add_dune_ptscotch_flags macro.

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