Move Nix files into a nix directory

Move everything from `config` to the root level.

nvim/plugin/vim-test.lua

@@ -0,0 +1,23 @@
+local map = vim.api.nvim_set_keymap
+
+local options = {
+  silent = true,
+}
+
+map("n", "<leader>tf", ":TestFile<CR>", options)
+map("n", "<leader>tg", ":TestVisit<CR>", options)
+map("n", "<leader>tl", ":TestLast<CR>", options)
+map("n", "<leader>tn", ":TestNearest<CR>", options)
+map("n", "<leader>ts", ":TestSuite<CR>", options)
+
+vim.cmd [[
+  let test#echo_command = 0
+  let test#strategy = "neovim_sticky"
+
+  let g:test#neovim_sticky#kill_previous = 1
+  let g:test#neovim_sticky#reopen_window = 1
+  let g:test#preserve_screen = 0
+
+  let test#php#phpunit#executable = './run test'
+  let test#php#phpunit#options = '--colors=always --testdox'
+]]