Hopefully fix sporadic test failures

test_booting_a_foreground_server was sometimes failing on the CI, I
think because the pidfile can be created before the socket file exists,
and therefore before the server has fully booted.

Author: jonleighton

lib/spring/test/acceptance_test.rb

@@ -519,7 +519,7 @@ def exec_name
           app.run "spring server &"
 
           Timeout.timeout(1) do
-            sleep 0.1 until spring_env.server_running?
+            sleep 0.1 until spring_env.server_running? && spring_env.socket_path.exist?
           end
 
           assert_success app.spring_test_command