Fix tests dealing with retryable bodies.

Test them in a slightly more robust manner.
- Rely on the fallback from Modern TLS for the retry, instead
  of a fallback due to a closed socket.
- Use mode DISCONNECT_AFTER_READING_RESPONSE to make sure the
  response is consumed before being discarded.

Change-Id: I8227148e1fc04b7557a41ee6485ffe98efb77ace

Conflicts:
	luni/src/test/java/libcore/java/net/URLConnectionTest.java