[LLDB] On Windows, fix tests

This patch fixes a few issues seen when running `ninja check-lldb` in a Release build with VS2017:

- Some binaries couldn't be found (such as lldb-vscode.exe), because .exe wasn't appended to the file name.
- Many tests used to fail since our installed locale is in French - the OS error messages are not emitted in English.
- Our codepage being Windows-1252, python failed to decode some error messages with accentuations.

Differential Revision: https://reviews.llvm.org/D88975