Replace vim.lsp.buf.server_ready in tests.

It's deprecated and won't appear in neovim 0.10.

See neovim/neovim#23520

We're trying to make sure we get a response here, so just
call some other LSP method and see, though we may be able to
remove this entirely.

lua/tests/helpers.lua

@@ -79,7 +79,9 @@ function helpers.wait_for_loading_pins(iv)
 end
 
 function helpers.wait_for_ready_lsp()
-  local succeeded, _ = vim.wait(15000, vim.lsp.buf.server_ready)
+  local succeeded, _ = vim.wait(15000, function()
+    return not not vim.lsp.buf_notify(0, '$/progress', {})
+  end)
   assert.message('LSP server was never ready.').True(succeeded)
 end