Problem:
Tests that need to check `nvim` CLI behavior (no RPC session) create
their own ad-hoc `system()` wrappers.
Solution:
- Use `n.spawn_wait` instead of `system()`.
- Bonus: this also improves the tests by explicitly checking for
`stdout` or `stderr`. And if a signal is raised, `ProcStream.status`
will reflect it.
Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.
Closes https://github.com/neovim/neovim/issues/27004.
Previously, if the autocommands are not triggered, the tests may still
pass because no assertion is done. Add an assertion so that the tests
will fail if the autocommands aren't triggered.
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- `redir_exec` is obsolete, but it keeps getting used in new tests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`.
