Problem
- `redir_exec` is obsolete, but it keeps getting used in new tests
  because people copy existing tests.
- Disadvantages of `redir_exec`:
  - Captures extra junk before the actual error/message that we _want_ to test.
  - Does not fail on error, unlike e.g. `command()`.

Solution
- Use new functions like `nvim_exec` and `pcall_err`.

It's possible for weirdness to happen if curbuf is modified while
sourcing from it via :source (with no arguments). For example:

- Deleting lines from or wiping curbuf can cause internal error E315 to
  be thrown from ml_get.
- Changing the curbuf to another buffer while sourcing can cause lines
  from the new curbuf to then be sourced instead.

Problem:
Anonymous :source (no args) and nvim_exec() don't support Vimscript line continuations.

Solution:
Factor out the concat logic into concat_continued_line() and a
CONCAT_CONTINUED_LINES macro for simple concatenations where lines are
fetched individually.

Closes #14807

Problem:    When 'hidden' is set session creates extra buffers.
Solution:   Move :badd commands to the end. (Jason Franklin)
https://github.com/vim/vim/commit/d39e275b57493f9e25e1b62f84810571eee30cf4

Adjust some tests in ex_cmds/mksession_spec.lua:

- 'restores same :terminal buf in splits': Buffers aren't always :badded
  in the same order as they're :edited, :balted, etc, so the order of
  buffers in the buffer list may change slightly now that :badd happens
  afterwards.
- 'restores buffers with tab-local CWD': This is explained in a comment.

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

It's happening because do_source is only expected to return FAIL when it
was unable to open file . But `nlua_exec_file` returns fail for parsing
and execution error too . Those errors are emitted through `nlua_error`.

So now return value of nlua_exec_file is ignored like do_cmdline. It now
only returns fail when it was unable to open file that check is done
before calling nlua_exec_file or do_cmdline. Errors in nlua_exec_file
are still directly emitted through nlua_error like before.