Refactor sample consensus tests

Travis started to routinely crash while building `test_sample_consensus`. The test covers all of the components of the module and therefore its compilation involves instantiation of a huge number of templates. I hypothesize that this is the reason for crashes.

This pull request rearranges and cleans up tests for `sample_consensus` module. Tests are split into four parts to reduce the number of template instantiations in each particular test file. Additional clean-up was performed, see commit messages for an overview.