Now that it even works at all (type = 0 bug aside), add tests for it.
Test both close_notify being received before and after SSL_shutdown is
called. In the latter case, have the peer send some junk to be ignored
to test that works.
Also test that SSL_shutdown fails on unclean shutdown and that quiet
shutdowns ignore it.
BUG=526437
Change-Id: Iff13b08feb03e82f21ecab0c66d5f85aec256137
Reviewed-on: https://boringssl-review.googlesource.com/5769
Reviewed-by: Adam Langley <agl@google.com>