helpers.source() was a hack to work around the lack of anonymous
:source. Its "create tempfile" behavior is not a required part of most
tests that use it.

Some tests still need the old "create tempfile" behavior either because
they test SID behavior, or because of missing nvim_exec features: #16071

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- `redir_exec` is obsolete, but it keeps getting used in new tests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.
https://github.com/vim/vim/commit/461a7fcfce3cd6414f990037e6468af3b5ccf119

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.

Fixes #6954