test(harness): migrate away from magic globals

Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

test/functional/testnvim.lua

@@ -1167,22 +1167,13 @@ local testid = (function()
 end)()
 
 return function()
-  local g = getfenv(2)
-
-  --- @type function?
-  local before_each = g.before_each
-  --- @type function?
-  local after_each = g.after_each
-
-  if before_each then
-    before_each(function()
+  if harness.is_defining() then
+    harness.before_each(function()
       local id = ('T%d'):format(testid())
       _G._nvim_test_id = id
     end)
-  end
 
-  if after_each then
-    after_each(function()
+    harness.after_each(function()
       close_extra_sessions(_G._nvim_test_id, true)
       if not vim.endswith(_G._nvim_test_id, 'x') then
         -- Use a different test ID for skipped tests as well as Nvim instances spawned