Make Lua engine tests modular

Rework the LuaEngine tests to be entirely modular, so that every library
need not add itself to one monolithic file. This also allows other
extensions that add Lua modules to make unit tests without having to
somehow inject them into a test class owned by Scribunto.

The approach taken is similar to that used for Selenium for running
tests against multiple browsers.

Change-Id: I294b2a8195759c0e4fa211f879305a8eb66d9c9a