PORTS: Tests do not compile with Visual Studio

[elasota]

To reproduce run this:

cd dists\msvc
create_project ..\.. --use-canonical-lib-names --enable-text-console --msvc --tests

Open scummvm-tests.sln in Visual Studio and attempt to build Release.

Currently fails due to multiple issues:

• Python 3 defaults encoding to Windows-1252 and cxxtestgen.py fails to digest punycode.h due to it containing unrecognizable characters.
• Fails to link due to missing Wintermute and Ultima symbols (happens even when Ultima and Wintermute are disabled)
• Fails to link due to missing "install_null_g_system" symbol, due to null_osystem.cpp not being included
• Fails to compile null_osystem.cpp if it is included due to include paths like "common/array.h" being resolved relative to the source file directory at higher priority than the "scummvm" root directory, causing it to include recursively

[digitall]

The first part of this bug i.e. python 3 issue with codepage has been fixed by merging of ​https://github.com/scummvm/scummvm/pull/3824 supplied by elasota.