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/functions_spec.lua

@@ -0,0 +1,20 @@
+-- Tests for misc Vimscript |functions|.
+--
+-- If a function is non-trivial, consider moving its spec to:
+--    test/functional/vimscript/<funcname>_spec.lua
+--
+-- Core "eval" tests live in eval_spec.lua.
+
+local helpers = require('test.functional.helpers')(after_each)
+
+local clear = helpers.clear
+local eval = helpers.eval
+local iswin = helpers.iswin
+local matches = helpers.matches
+
+before_each(clear)
+
+it('windowsversion()', function()
+  clear()
+  matches(iswin() and '^%d+%.%d+$' or '^$', eval('windowsversion()'))
+end)