Add Polylib unit tests

Issue/feature addressed

Currently the Polylib unit tests are stored in a standalone, non-compiled file Polylib_test.cpp. This MR adds them to the UnitTest framework.

Proposed solution

See above.

Implementation

The tests have been ported to use Boost.Test, and have been lightly refactored to remove duplicate code using std::function.

Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).