Update Lua unit tests to lua-TestMore (with TAP output)

This pull request updates the Lua unit tests in `src/bindings/lua/tests` to use [lua-TestMore] instead of lunit. lua-TestMore not only generates TAP output, as our other tests now do, but
also allows tests to run stand-alone (i.e. not under `lunit` driver), so works around the issue
reported in #36.

lua-TestMore is an extensible TAP framework for lua, so it will hopefully be useful for system
and integration testing as well.

[lua-TestMore]: https://fperrad.github.io/lua-TestMore/