REGTESTS: converter: serve the two sha2 requests on one connection

REGTESTS: converter: serve the two sha2 requests on one connection

The final validation campaign caught this test with "HTTP header is
incomplete" when the whole suite runs with "--j 32" on a loaded
machine. Its client sends its two requests over a single connection
while the server answers each of them in its own iteration, so it
closes after the first response, exactly when the second request is
sent over the connection haproxy reuses, and the client gets that close
instead of a response.

This is the same shape which was fixed in the transaction state test:
the server now keeps its connection for both requests. Twenty runs in
parallel with the machine loaded pass.