Add tests for each file encoding supported by VS Code

When a file is open in the editor, VS Code decodes it and provide extensions with UTF-8 content. When a header is read directly from disk, cpptools must do the decoding.

While investigating an issue with GB2312, I discovered that the encoding "EUC-CN" from iconv (GB2313 v. A1A4?) is not the same version used by VS Code itself (v. A1AA?) for that encoding. (See: https://en.wikipedia.org/wiki/GB_2312 , "Two implementations of GB2312"). Since both use iconv, I assume VS Code is using the "GBK" encoding for both GBK and GB2312 (as GB2313 is a subset of GBK).

I'm concerned that other encodings we support may have similar nuances. Opening this issue to track adding tests that exercise cpptools decoding, for all file encodings supported by VS Code.