test(harness): avoid overlong socket filename #41488

Problem:
Test sockets live under `$TMPDIR`, which the harness points at the build
dir.  On macOS/BSD `sockaddr_un.sun_path` is 104 bytes, and a CI build
path plus "nvim.<pid>.<n>" leaves little room:

    /Users/runner/work/neovim/neovim/build/Xtest_tmpdir_terminal/nvim.runner/aBcDeF/nvim.12345.0

Solution:
Point XDG_RUNTIME_DIR (`stdpath('run')`) at "/tmp/nvim_<pid>".  28 bytes:

    /tmp/nvim_19916/nvim.19919.1

TODO?:
- `TEMP_DIR_NAMES` prefers `$TMPDIR` over `/tmp`, so on macOS
  `stdpath('run')` defaults to the long `/var/folders/<xx>/<…>/T/` path
  instead of the short `/tmp` alias.

Note:
- The 104-byte limit applies to the `bind()` arg, not its "realpath",
  to, so `/tmp/…` symlinks can be used to workaround the limit.

test/functional/testterm.lua

@@ -206,6 +206,8 @@ function M.setup_child_nvim(args, opts)
   local env = t.shallowcopy(opts.env) or {}
   env.VIMRUNTIME = env.VIMRUNTIME or os.getenv('VIMRUNTIME')
   env.NVIM_TEST = env.NVIM_TEST or os.getenv('NVIM_TEST')
+  -- Child servers need the socket dir set by runner.lua.
+  env.XDG_RUNTIME_DIR = env.XDG_RUNTIME_DIR or os.getenv('XDG_RUNTIME_DIR')
 
   return M.setup_screen(opts.extra_rows, argv, opts.cols, env)
 end