- Move os_name() up to "global helpers".
- Rename it to is_os().
- Make it depend on uname() instead of a running Nvim instance.

Problem:    The a: dict is not immutable as documented.
Solution:   Make the a:dict immutable, add a test. (Ozaki Kiichi, Yasuhiro
            Matsumoto, closes vim/vim#3929)
https://github.com/vim/vim/commit/31b816042fca879b11965ddd75287732563ba698

Problem:    ":args" output is hard to read.
Solution:   Make columns with the names if the output is more than one line.
https://github.com/vim/vim/commit/5d69da462f584a3aefb3427b127334bf9af3a4b0

vim-patch:8.0.1740: warning for signed-unsigned incompatibility

Problem:    Warning for signed-unsigned incompatibility.
Solution:   Change type from "char *" to "char_u *". (John Marriott)
https://github.com/vim/vim/commit/405dadb63ea2b7aa4c8c659807506a35a8a9504c

Removes ported legacy test that was re-added later.
Ref: https://github.com/neovim/neovim/pull/10147#issuecomment-512609513

Problem:    Expanding a file name "~" results in $HOME. (Aidan Shafran)
Solution:   Change "~" to "./~" before expanding. (closes vim/vim#3072)
https://github.com/vim/vim/commit/00136dc321586800986e8f743c2f108f5eecbf92

Problem:    Cannot test if a command causes a beep.
Solution:   Add assert_beeps().
https://github.com/vim/vim/commit/b48e96f61c87a64e38e3ac50732c92a84a4833b8

There is no need to call update_screen() directly in an API function,
mode input processing invokes update_screen() as needed. And if the API
call is done in a context where redraw is disabled, then redraw is
disabled for a reason. A lot of API functions are of equal semantical
strength (nvim_call_function and nvim_execute_lua can also do whatever,
nvim_command is not special), this inconsistency has no purpose.

Problem:    Packadd test fails on MS-Windows.
Solution:   Ignore difference between forward and backward slashes.
https://github.com/vim/vim/commit/53c8a478cc4265549597b00214e0da812154742e