Add primitive formspec testing feature

[y5nw]

This should at least partially address #53.

This PR is still WIP. Specifically, it does not provide an API to "interact" (*) with formspec elements, and formspecs need to be parsed in order to send the fields correctly.

(*) Whether the specific element is present is a different matter. IMO not checking this allows mods to test whether the callback handles certain situations correctly, e.g. formspecs where some parts are conditionally hidden but have only one callback function (which also need to have the code to make sure certain fields are not handled when they are not supposed to be).

[S-S-X]

I've done some work with formspec testing, unfortunately I've messed up local dev and have not pushed anything here in long time. Trying to get it sorted out... some day.

I'll try to merge this PR with what I've done so far (formspec parsing, looking for elements, player formspec, etc.).

[y5nw]

Oops, looks like I have forgotten this PR. Admittedly it was too ambitious for my skills at the time so it ran out of steam quite quickly.

[S-S-X]

Did some rebasing in an attempt to mix in your work and to get somewhat clean history out of it, preview here:
https://github.com/S-S-X/mineunit/compare/v0.13.0...v0.14?expand=1

Howevert, your last commit Add primitive wrapper for quitting formspecs isn't there, others are in one form or another.

Things I should probably drop before making a release:

• Player._formspec name + content table, while going through rebases it was an attempt to keep also the way you did it but done that I'm pretty sure it should be dropped and just Form class instances should stay.
• Actual field and action handling form Form instances, my idea was basically this:

  mineunit/formspec.lua

  Lines 262 to 274 in 0db49bb

  	-- Get fields that would be submitted with any of the basic submit actions.
  	-- TODO: Allow specifying trigger, like button or scrollbar for example, and return fields based on action.
  	-- FIXME: Currently results will be wrong: includes everything like buttons that shouldn't be there.
  	function Form:fields()
  	mineunit:error("Form:fields() is experimental and UNSTABLE. Its behavior and interface WILL BE CHANGED.")
  	local results = {}
  	for e in ivalues(self._data) do
  	if submit_fields[e:type()] and e:name() ~= "" then
  	results[e:name()] = e:value()
  	end
  	end
  	return results
  	end

• Maybe clean up tests a bit...