feat: run tests independent from user's nvim config

.gitignore

···
-playground/
+/playground/
+/.tests/

Taskfile.yml

···
     cmds:
       - |
         nvim --headless \
-          -u ./spec/minimal_init.vim\
+          -u ./spec/minimal_init.lua\
           -c "PlenaryBustedDirectory spec \
               {minimal_init='./spec/minimal_init.lua'\
               ,sequential=true}"

spec/minimal_init.lua

···
+local function root(p)
+  local f = debug.getinfo(1, "S").source:sub(2)
+  return vim.fn.fnamemodify(f, ":p:h:h") .. "/" .. (p or "")
+end
+
+local function install_plug(plugin)
+  local name = plugin:match ".*/(.*)"
+  local package_root = root ".tests/site/pack/deps/start/"
+  if not vim.loop.fs_stat(package_root .. name) then
+    print("Installing " .. plugin)
+    vim.fn.mkdir(package_root, "p")
+    vim.fn.system {
+      "git",
+      "clone",
+      "--depth=1",
+      "https://github.com/" .. plugin .. ".git",
+      package_root .. "/" .. name,
+    }
+  end
+end
+
+vim.cmd [[set runtimepath=$VIMRUNTIME]]
+vim.opt.runtimepath:append(root())
+vim.opt.packpath = { root ".tests/site" }
+vim.notify = print
+
+install_plug "nvim-lua/plenary.nvim"
+install_plug "nvim-treesitter/nvim-treesitter"
+
+vim.env.XDG_CONFIG_HOME = root ".tests/config"
+vim.env.XDG_DATA_HOME = root ".tests/data"
+vim.env.XDG_STATE_HOME = root ".tests/state"
+vim.env.XDG_CACHE_HOME = root ".tests/cache"

spec/minimal_init.vim

···
-set rtp+=.
-packadd plenary.nvim
-packadd nvim-treesitter
-packadd nvim-dap