Document VSTest pitfalls and differences from MTP for input encoding

[Youssef1313]

When starting a process from a console, it gets its input encoding based on the console codepage. All child processes then inherit the same input encoding, except if a child process is run with CreateNoWindow = true.

With MTP, we don't use CreateNoWindow in any place.

With VSTest, it depends.

• When running with isolation, vstest.console.exe starts testhost.exe with CreateNoWindow=true, and so input encoding is lost.
• When running without isolation, everything is run under vstest.console.exe, and there are no child processes at all, and original encoding is preserved.

Most VSTest users will run with isolation, and might hit this behavior difference when migrating to MTP.

This needs to be clearly documented.