test(refactor): remove invalid test plugins

- `popup.nvim` is not referenced anywhere within the plugin and as such
  should be removed.
- `playground` is not needed either, it's not referenced and secondly
  the most important feature from it (:TSPlayground) is covered by
  Neovim's :InspectTree command in more recent versions of Neovim.

Closes #203 (#203)

tests/minimal_init.lua

@@ -138,7 +138,5 @@ end
 
 M.setup({
     ["plenary.nvim"] = "https://github.com/nvim-lua/plenary.nvim",
-    ["popup.nvim"] = "https://github.com/nvim-lua/popup.nvim",
     ["nvim-treesitter"] = "https://github.com/nvim-treesitter/nvim-treesitter",
-    ["playground"] = "https://github.com/nvim-treesitter/playground",
 })