Add test to check when proxy times-out mid-write to upstream

I've add to add code to allow the environment to specify the upstream
tiemout so we don't have to wait 30s for this test to happen.

tests/acceptance/flexnbd/fake_dest.rb

@@ -54,6 +54,10 @@ module FlexNBD
         write_reply(handle, 1)
       end
 
+      def nread
+        @sock.nread
+      end
+
       def disconnected?
         Timeout.timeout(2) do
           @sock.read(1).nil?
@@ -85,6 +89,10 @@ module FlexNBD
         @sock.write(len)
       end
 
+      def getsockopt(level, optname)
+        @sock.getsockopt(level, optname)
+      end
+
       def self.parse_be64(str)
         raise "String is the wrong length: 8 bytes expected (#{str.length} received)" unless
           str.length == 8