testutils: fix test flake on HEAD request

[dbungert]

testutils: fix test flake on HEAD request

Flakiness may be observed when running the unit tests.
The symptom of that looks like:
requests.exceptions.ConnectionError: ('Connection aborted.',
BadStatusLine('Hello world!HTTP/1.1 200 OK\r\n'))

We believe that, with the correct send and recv timing, the "Hello
world!" body from a HEAD request will be left unread in the reused
socket, resulting in an invalid read for the next request along.

Co-authored-by: Olivier Gayot [email protected]

[dbungert]

Testing was performed with 39b9c64 cherry-picked to the branch but is otherwise independent of that change.