Test layer between hstox and zetox.

Additional docs for writing tests are at https://toktok.github.io/spec#test-protocol

Tests are ran by linking test executable under hstox/test/ directory and issuing cabal test.

Some setup for hstox and zetox is required. For more info, refer to their docs.

Running all tests is tad slow, thus one can run a single test with:

./dist/build/test-tox/test-tox -m <test name>

TODO: add travis