Improve dnd-character tests

[ryanplusplus]

This removes some unnecessary checks and fixes the argument order in an assertion.

[ryanplusplus]

The order must be expected, actual in order for the error message to be correct.

[ryanplusplus]

This check is unnecessary because strength is initialized with character.strength above.

[keiravillekode]

The problem-specifications say to check "each ability is only calculated once"

Here is a solution we want to reject:

function Character:new(name)
  self.__index = function(mytable, key)
    return ability(roll_dice())
  end

  local meta_tbl = {
    name = name,
    constitution = ability(roll_dice())
  }
  meta_tbl.hitpoints = 10 + modifier(meta_tbl.constitution)

  return setmetatable(meta_tbl, self)
end

Instead of three tests

      "description": "random ability is within range",
      "description": "random character is valid",
      "description": "each ability is only calculated once",

I wrote one test for each ability.

[ryanplusplus]

Thanks, I put them back.

[keiravillekode]

lgtm