Problem:    When using an assert function one can either specify a message or
            get a message about what failed, not both.
Solution:   Concatenate the error with the message.
https://github.com/vim/vim/commit/c7b831ca154537505f5a22d01335a86b2e9cb023

after_each + os.remove ensures Xfile is deleted after every test.
Windows exit code is inconsistent.

mkfifo (msysgit) does not work outside of msys2 environment.
gzip tests fail on Windows.

mklink requires admin privs for file symbolic links so mklink fails.

- Return VimL errors instead of generic errors for:
  - nvim_call_function
  - nvim_call_dict_function
- Fix tests which were silently broken before this change.

This violates #6150 where we agreed not to translate API errors.  But
that can be fixed later.

Try nvim's delete() for cross-platform file remove in Windows