Merge bitcoin/bitcoin#32408: tests: Expand HTTP coverage to assert libevent behavior

f16c8c67bf tests: Expand HTTP coverage to assert libevent behavior (Matthew Zipkin)

Pull request description:

  These commits are cherry-picked from #32061  and part of a project to [remove libevent](https://github.com/bitcoin/bitcoin/issues/31194).

  This PR only adds functional tests to `interface_http` to cover some HTTP server behaviors we inherit from libevent, in order to maintain those behaviors when we replace libevent with our own HTTP server.

  1. Pipelining: The server must respond to requests from a client in the order in which they were received [RFC 7230 6.3.2](https://www.rfc-editor.org/rfc/rfc7230#section-6.3.2)
  2. `-rpcservertimeout` config option which sets the amount of time the server will keep an idle client connection alive
  3. "Chunked" Transfer-Encoding: Allows a client to send a request in pieces, without the `Content-Length` header [RFC 7230 4.1](https://www.rfc-editor.org/rfc/rfc7230#section-4.1)

ACKs for top commit:
  achow101:
    ACK f16c8c67bf
  vasild:
    ACK f16c8c67bf
  polespinasa:
    ACK f16c8c67bf
  fjahr:
    utACK f16c8c67bf

Tree-SHA512: 405b59431b4d2bf118fde04b270865dee06ef980ab120d9cc1dce28e5d65dfd880a57055b407009d22f4de614bc3eebdb3e203bcd39e86cb14fbfd62195ed06a

test/functional/test_framework/test_node.py

@@ -154,6 +154,7 @@ class TestNode():
         self.process = None
         self.rpc_connected = False
         self.rpc = None
+        self.reuse_http_connections = True # Must be set before calling get_rpc_proxy() i.e. before restarting node
         self.url = None
         self.log = logging.getLogger('TestFramework.node%d' % i)
         # Cache perf subprocesses here by their data output filename.
@@ -285,6 +286,7 @@ class TestNode():
                     timeout=self.rpc_timeout // 2,  # Shorter timeout to allow for one retry in case of ETIMEDOUT
                     coveragedir=self.coverage_dir,
                 )
+                rpc.auth_service_proxy_instance.reuse_http_connections = self.reuse_http_connections
                 rpc.getblockcount()
                 # If the call to getblockcount() succeeds then the RPC connection is up
                 if self.version_is_at_least(190000) and wait_for_import: