fix test/test_unix_socket.rb (#1870)

Fix to work reliably on platforms without UNIXSockets

test/test_unix_socket.rb

@@ -9,17 +9,19 @@ class TestPumaUnixSocket < Minitest::Test
   Path = "test/puma.sock"
 
   def setup
-    skip UNIX_SKT_MSG unless UNIX_SKT_EXIST
+    return unless UNIX_SKT_EXIST
     @server = Puma::Server.new App
     @server.add_unix_listener Path
     @server.run
   end
 
   def teardown
+    return unless UNIX_SKT_EXIST
     @server.stop(true)
   end
 
   def test_server
+    skip UNIX_SKT_MSG unless UNIX_SKT_EXIST
     sock = UNIXSocket.new Path
 
     sock << "GET / HTTP/1.0\r\nHost: blah.com\r\n\r\n"