Use only a single clear() call in some test/functional/vimscript/ test
files whose test cases have very little side effect.
A downside of using a single clear() is that if a crash happens in one
test case, all following test cases in the same file will also fail, but
these functionalities and tests don't change very often.
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.
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- `redir_exec` is obsolete, but it keeps getting used in new tests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:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason. Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).
Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
- Move or merge `…/visual/*` and `…/insert/*` tests into here where
appropriate.
- Rename `…/eval/` to `…/vimscript/`.
- Move `…/viml/*` into here also.
* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua
