test: reorg #15698

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

test/functional/vimscript/vvar_event_spec.lua

@@ -0,0 +1,15 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, eval, eq = helpers.clear, helpers.eval, helpers.eq
+local command = helpers.command
+describe('v:event', function()
+  before_each(clear)
+  it('is empty before any autocommand', function()
+    eq({}, eval('v:event'))
+  end)
+
+  it('is immutable', function()
+    eq(false, pcall(command, 'let v:event = {}'))
+    eq(false, pcall(command, 'let v:event.mykey = {}'))
+  end)
+end)
+