Use known cookie when starting runtimes (#199)

.github/workflows/test.yaml

@@ -19,7 +19,7 @@ jobs:
       - name: Check formatting
         run: mix format --check-formatted
       - name: Run tests
-        run: mix test
+        run: elixir --cookie "COOKIEFORTESTS" -S mix test
       - name: Install Node
         uses: actions/setup-node@v2
         with:

lib/livebook/runtime/standalone_init.ex

@@ -54,7 +54,10 @@ defmodule Livebook.Runtime.StandaloneInit do
       # Enable ANSI escape codes as we handle them with HTML.
       "+sbwt none +sbwtdcpu none +sbwtdio none -elixir ansi_enabled true",
       # Make the node hidden, so it doesn't automatically join the cluster
-      "--hidden"
+      "--hidden",
+      # Use the cookie in Livebook
+      "--cookie",
+      Atom.to_string(Node.get_cookie())
     ]
   end