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/hostname_spec.lua

@@ -0,0 +1,20 @@
+local helpers = require('test.functional.helpers')(after_each)
+local eq = helpers.eq
+local ok = helpers.ok
+local call = helpers.call
+local clear = helpers.clear
+local iswin = helpers.iswin
+
+describe('hostname()', function()
+  before_each(clear)
+
+  it('returns hostname string', function()
+    local actual = call('hostname')
+    ok(string.len(actual) > 0)
+    if call('executable', 'hostname') == 1 then
+      local expected = string.gsub(call('system', 'hostname'), '[\n\r]', '')
+      eq((iswin() and expected:upper() or expected),
+         (iswin() and actual:upper() or actual))
+    end
+  end)
+end)