To test the curl sources it is very useful to have a small httpd
server that can serve an arbitrary directory. This helper will
ensure that via:
```python
with with osbuild.testutil.net.http_serve_directory(fake_httpd_root) as httpd:
port = httpd.server_port
# download from http://localhost:{port}/<any-path-under-httpd-root>
```
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