Add missing stdint.h includes in unit tests files

Add missing stdint.h includes in unit tests files

My mingw compiler/headers (mingw-w64 10.0.0 on macOS) seem to be more
pendantic than the one that comes with Ubuntu 22.04 (github actions) or
any of the other platforms including msvc/normal windows header.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20230208001819.244694-5-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26182.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>