This also adds two test utilities from dune-functions
that are both used in this test.

* TestSuite: A helper class to organize checks in tests
  in a unified way and more readable way.
* CollectorStream: A simple stream that allows to collect
  data and forward it to its creator using a callback.
  Used by testsuite.hh

Since can also be used for tests in other modules they
are installed in dune/common/test. However, CollectorStream
is not strictly related to tests and may also be moved directly
to dune/common/.