mbuf: Add unit tests

mbuf: Add unit tests

While fixing the conversion warning I was
somewhat confused how this works, so added
UTs to verify I understood it.

v2:
 - disable assert test for MS VS
 - add define for memory-intensive UTs and
   only enable it by default for CMake builds,
   so we do not break a lot of builds out there
   due to memory allocation failures

Change-Id: Icab68a5fd1b6288955f0073179f1ddde1468d951
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1432
Message-Id: <20251212120352.17402-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg35050.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>