Add building unit tests with mingw to github actions

Add building unit tests with mingw to github actions

This runs each test in its own action since order of stderr and stdout
is seemingly random in github action Windows output and this way at least
tests outputs are grouped by test

Patch v2: use -static-libgcc to avoid comping gcc runtime libraries.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20230209163705.466173-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26204.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>