Ensure that
  * Shell uses UTF8 input/output mode
  * Stderr output is captured, in UTF8
  * Program exit codes are correctly captured

Update functional test harness and add tests
for :make command.

Closes #13713

Problem:    Cannot use "q" to cancel a number prompt.
Solution:   Recognize "q" instead of ignoring it.
https://github.com/vim/vim/commit/eebd555733491cb55b9f30fe28772c0fd0ebacf7

Problem:    Line() only works for the current window.
Solution:   Add an optional argument for the window to use.
https://github.com/vim/vim/commit/8e0a8e7eb7c177807f44db6b76d8e52314248ab5

Replacement for Vim's test_null_string().
Vim uses it to verify that its codebase handles null strings.
Preparation for the Test_null_list() in patch v8.2.1822.

Use v:_null_string, not non-existent env var, for null string tests.

Mention v:_null_string in id() because id(v:_null_string) returns (nil).

Close https://github.com/neovim/neovim/issues/8614

Use uv_os_homedir and current directory as backup if HOME env is not set.
Allocate homedir only if var is not NULL.

This reverts commit 0519a75f6eca1065a4d0184f99c71ae03a99b9b1.

Problem:    Peeking and flushing output slows down execution.
Solution:   Do not update the mode message when global_busy is set.  Do not
            flush when only peeking for a character. (Ken Takata)
https://github.com/vim/vim/commit/cb574f415486adff645ce384979bfecf27f5be8c