[gsoc2019] Add tests for adaptive thresholding

@miralshah365 has submitted implementation of the algorithm in #341 with example-as-test, which has worked fine for development, but we need more complete test suite for it.

We need a basic variety of input images.

### References

Test images options are being discused arranged:
- PNM format is going to move to the core features to be offered as built-in, see [[RFC] Managing input images for tests"](https://lists.boost.org/boost-gil/2019/06/0252.php)
- Hosting of standard images for IO tests (and not only), see [[IO] Hosting test images](https://lists.boost.org/boost-gil/2019/07/0279.php)