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- `redir_exec` is obsolete, but it keeps getting used in new tests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`.
Problem: :echomsg and :echoerr do not handle List and Dict like :echo does.
(Daniel Hahler)
Solution: Be more tolerant about the expression result type.
461a7fcfce
Add lua functional tests for :echo,:echon,:echomsg,:echoerr
because nvim did not port "test_" functions from Vim
that modify internal state.
Testing :echoerr via try/catch is sufficient.
