tests: add test case using self

See #756.

spec/subtyping/self_spec.lua

@@ -0,0 +1,43 @@
+local util = require("spec.util")
+
+describe("subtyping of self", function()
+   it("self type resolves from abstract interface to concrete records (#756)", util.check([[
+      local interface SoundMaker
+         make_sound: function(self)
+      end
+
+      local record Animal is SoundMaker
+         species: string
+      end
+
+      function Animal:create(species: string): Animal
+         return setmetatable({ species = species }, { __index = Animal })
+      end
+
+      function Animal:make_sound()
+         print("Animal sound")
+      end
+
+      local record Person is SoundMaker
+         name: string
+      end
+
+      function Person:create(name: string): Person
+         return setmetatable({ name = name }, { __index = Person })
+      end
+
+      function Person:make_sound()
+         print("Person sound")
+      end
+
+      local things: {SoundMaker} = {
+         Animal:create("Dog"),
+         Person:create("John")
+      }
+
+      for _, thing in ipairs(things) do
+         thing:make_sound()
+      end
+   ]]))
+end)
+