buffer_list_aggregate_separator(): add unit tests

buffer_list_aggregate_separator(): add unit tests

Before refactoring this function, add tests to verify the new
implementation.

While writing the tests, it became clear that this function is not
behaving very well.  We'll fix that in follow-up commits.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20171104224551.3079-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15748.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 2ddb527abe38f5866ff01e91f8ee89d0f9700762)