test/test_pair: make TestPairM#test_write_nonblock_retry faster

Write 4099-bytes blocks instead of 11-bytes blocks to run it faster. The
buffer may be as large as megabytes and it takes too much time to fill
up, especially under GC.stress. I didn't measured but it didn't finish
in an hour.

test/test_pair.rb

@@ -295,7 +295,7 @@ def test_write_nonblock_retry
       # fill up a socket so we hit EAGAIN
       written = String.new
       n = 0
-      buf = 'a' * 11
+      buf = 'a' * 4099
       case ret = s1.write_nonblock(buf, exception: false)
       when :wait_readable then break
       when :wait_writable then break