fix: don't crawl cwd when adding a new buffer

[stevearc]

If you recall a long time ago I advocated for the option to set discovery.enabled = false because I work with a large repo and crawling it for tests causes Neovim to be nearly frozen for 20+ minutes while pegging the CPU. I noticed this behavior again recently, and tracked it down to this:

neotest/lua/neotest/client/init.lua

Lines 393 to 398 in 32ff2ac

	autocmd({ "BufAdd", "BufWritePost" }, function(ev)
	local file_path = vim.fn.fnamemodify(ev.file, ":p")
	if not lib.files.exists(file_path) then
	return
	end

For me, this was happening when I opened the quickfix, but the key is just that BufAdd was getting triggered for a non-file buffer. This lead to ev.file being an empty string, and when you pass that in to fnamemodify you get vim.fn.fnamemodify("", ":p") which returns your current directory. It looks like the intent of this autocmd is to check for changes to existing files or creation of new files and re-searching the parent directory for tests. What was actually happening was we were passing that file_path into self._update_positions lower down in that function

neotest/lua/neotest/client/init.lua

Line 438 in 32ff2ac

self:_update_positions(file_path, { adapter = adapter_id })

Which resulted in a recursive search originating from the current working directory.

The fix that I opted to put in here was just to early return if the ev.file is an empty string, since it seems like neotest probably shouldn't care about those buffers.