wininet: Add tests for asynchronous HttpSendRequestEx/HttpEndRequest.

[Nicolas Le Cam]

Hi Hans,

Your patch makes the test timeout on my machine
(http://test.winehq.org/data/f212579ae9a1b770ebd34cec20f95e1977bb57f0/2000_w2...).

Timeout occurs in HttpSendRequestEx_test on HttpEndRequest call which
hang. Disabling test_async_HttpSendRequestEx makes it pass again.

After some tests it seems to be a timing issue, but I haven't been
able to find a real solution. Just that waiting at least 500msec
between the two HttpEndRequest, or waiting at least 150ms after the
WaitForSingleObject that follow the second HttpEndRequest makes test
pass. Reading response doesn't change anything.

After reading a little bit about how this works I don't see anything
wrong. Perhaps you will.

thanks,
Nicolas Le Cam