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/editor/mode_visual_spec.lua

@@ -0,0 +1,26 @@
+-- Visual-mode tests.
+
+local helpers = require('test.functional.helpers')(after_each)
+local clear = helpers.clear
+local eq = helpers.eq
+local eval = helpers.eval
+local expect = helpers.expect
+local feed = helpers.feed
+local meths = helpers.meths
+
+describe('visual-mode', function()
+  before_each(clear)
+
+  it("select-mode Ctrl-O doesn't cancel Ctrl-O mode when processing event #15688", function()
+    feed('iHello World<esc>gh<c-o>')
+    eq({mode='vs', blocking=false}, meths.get_mode()) -- fast event
+    eq(2, eval('1+1'))  -- causes K_EVENT key
+    eq({mode='vs', blocking=false}, meths.get_mode()) -- still in ctrl-o mode
+    feed('^')
+    eq({mode='s', blocking=false}, meths.get_mode()) -- left ctrl-o mode
+    feed('h')
+    eq({mode='i', blocking=false}, meths.get_mode()) -- entered insert mode
+    expect('h') -- selection is the whole line and is replaced
+  end)
+end)
+