In an attempt to repeat the problem in bug report #1850730

(http://curl.haxx.se/bug/view.cgi?id=1850730) I wrote up test case 552. The
test is doing a 70K POST with a read callback and an ioctl callback over a
proxy requiring Digest auth. The test case code is more or less identical to
the test recipe code provided by Spacen Jasset (who submitted the bug report).