feat(logging): include test-id in log messages

Problem:
1. Log messages (especially in CI) are hard to correlate with tests.
2. Since b353a5c05f #11886, dumplog() prints the logs next to test
   failures. This is noisy and gets in the way of the test results.

Solution:
1. Associate an incrementing id with each test and include it in log
   messages.
    - FUTURE: add v:name so Nvim instances can be formally "named"?
2. Mention "child" in log messages if the current Nvim is a child (based
   on the presence of $NVIM).

BEFORE:

    DBG … 12345      UI: event
    DBG … 12345      log_server_msg:722: RPC ->ch 1: …
    DBG … 12345      UI: flush
    DBG … 12345      inbuf_poll:444: blocking... events_enabled=1 events_pending=0
    DBG … 23454      UI: stop
    INF … 23454      os_exit:594: Nvim exit: 0

AFTER:

    DBG … T57        UI: event
    DBG … T57        log_server_msg:722: RPC ->ch 1: …
    DBG … T57        UI: flush
    DBG … T57        inbuf_poll:444: blocking... events_enabled=1 events_pending=0
    DBG … T57/child  UI: stop
    INF … T57/child  os_exit:594: Nvim exit: 0

test/functional/preload.lua

@@ -1,8 +1,9 @@
--- Modules loaded here will not be cleared and reloaded by Busted.
+-- Modules loaded here will NOT be cleared and reloaded by Busted.
 -- Busted started doing this to help provide more isolation.  See issue #62
 -- for more information about this.
 local helpers = require('test.functional.helpers')(nil)
 local iswin = helpers.iswin
+local busted = require("busted")
 
 if iswin() then
   local ffi = require('ffi')
@@ -12,3 +13,28 @@ if iswin() then
   ]]
   ffi.C._set_fmode(0x8000)
 end
+
+local testid = (function()
+  local id = 0
+  return (function()
+    id = id + 1
+    return id
+  end)
+end)()
+
+-- Global before_each. https://github.com/Olivine-Labs/busted/issues/613
+local function before_each(_element, _parent)
+  local id = ('T%d'):format(testid())
+  _G._nvim_test_id = id
+  return nil, true
+end
+busted.subscribe({ 'test', 'start' },
+  before_each,
+  {
+    -- Ensure our --helper is handled before --output (see busted/runner.lua).
+    priority = 1,
+    -- Don't generate a test-id for skipped tests. /shrug
+    predicate = function (element, _, status)
+      return not ((element.descriptor == 'pending' or status == 'pending'))
+    end
+  })