Added test for FUA acceptance.

Although I think this might be a bit useless as servers normally just
ingore flags.

tests/acceptance/test_serve_mode.rb

@@ -1,7 +1,6 @@
 require 'test/unit'
 require 'environment'
 require 'flexnbd/fake_source'
-require 'pp'
 
 class TestServeMode < Test::Unit::TestCase
   def setup
@@ -118,4 +117,14 @@ class TestServeMode < Test::Unit::TestCase
       assert_equal 0, rsp[:error]
     end
   end
+
+  def test_write_with_fua_is_accepted
+    connect_to_server do |client|
+      # Start with a file of all-zeroes.
+      client.write_with_fua(0, "\x00" * @env.file1.size)
+      rsp = client.read_response
+      assert_equal FlexNBD::REPLY_MAGIC, rsp[:magic]
+      assert_equal 0, rsp[:error]
+    end
+  end
 end