Tests: test_settings_send_timeout improved.

Data length adjusts depending on socket buffer size when it's possible.

test/python/body_generate/wsgi.py

@@ -0,0 +1,6 @@
+def application(env, start_response):
+    length = env.get('HTTP_X_LENGTH', '10')
+    bytes = b'X' * int(length)
+
+    start_response('200', [('Content-Length', length)])
+    return [bytes]