Added unittest + basic python and lua unittests (#174)

---------

Co-authored-by: Danymat <d.danymat@gmail.com>

tests/minimal_init.lua

@@ -0,0 +1,27 @@
+local plugins = {
+    ["https://github.com/nvim-treesitter/nvim-treesitter"] = os.getenv("NVIM_TREESITTER_DIRECTORY")
+        or "/tmp/nvim_treesitter",
+    ["https://github.com/nvim-lua/plenary.nvim"] = os.getenv("PLENARY_DIRECTORY") or "/tmp/plenary.nvim",
+}
+
+local tests_directory = vim.fn.fnamemodify(vim.fn.expand("<sfile>"), ":h")
+local plugin_root = vim.fn.fnamemodify(tests_directory, ":h")
+vim.opt.runtimepath:append(plugin_root)
+
+for url, directory in pairs(plugins) do
+    if vim.fn.isdirectory(directory) == 0 then
+        vim.fn.system({ "git", "clone", url, directory })
+    end
+
+    vim.opt.runtimepath:append(directory)
+end
+
+vim.cmd("runtime plugin/plenary.vim")
+require("plenary.busted")
+
+vim.cmd("runtime plugin/nvim-treesitter.lua")
+
+-- Some tests require the Python parser
+vim.cmd([[TSInstallSync! python]])
+
+require("neogen").setup({ snippet_engine = "nvim" })