test: zig build thread_spec.lua failure after `vim.text.diff` rename

Problem:
Since renaming `vim.diff` to `vim.text.diff`, `thread_spec.lua` fails in
the zig build. Is `vim.text.diff` not available in the thread context?
Why does it only fail in the zig build?

    FAILED   ./test/functional/lua/thread_spec.lua @ 217: thread vim.* diff
    ./test/functional/lua/thread_spec.lua:229: Expected objects to be the same.
    Passed in:
    (nil)
    Expected:
    (table: 0x7f221d392218) {
      [1] = 'notification'
    E5113: Lua chunk:
      [2] = 'result'
      [3] = {
        [1] = '@@ -1 +1 @@
    -Hello
    +Helli
    ' } }

    stack traceback:
            ./test/functional/lua/thread_spec.lua:229: in function <./test/functional/lua/thread_spec.lua:217>

    FAILED   ./test/functional/lua/thread_spec.lua @ 372: threadpool vim.* work
    ./test/functional/lua/thread_spec.lua:384: Expected objects to be the same.
    Passed in:
    (table: 0x7f2225be2c30) {
      [1] = 'notification'
      [2] = 'result'
     *[3] = {
       *[1] = vim.NIL } }
    Expected:
    (table: 0x7f2225be25c0) {
      [1] = 'notification'
      [2] = 'result'
     *[3] = {
       *[1] = '@@ -1 +1 @@
    -Hello
    +Helli
    ' } }

    stack traceback:
            ./test/functional/lua/thread_spec.lua:384: in function <./test/functional/lua/thread_spec.lua:372>

Solution:
Use `vim._diff` in the test, until a root cause is found.

1 files changed, 2 insertions, 2 deletions

diff --git a/test/functional/lua/thread_spec.lua b/test/functional/lua/thread_spec.lua
index 8653377760..9ff6b839f2 100644
--- a/test/functional/lua/thread_spec.lua
+++ b/test/functional/lua/thread_spec.lua
@@ -217,7 +217,7 @@ describe('thread', function()
     it('diff', function()
       exec_lua [[
         local entry = function(async)
-          async:send(vim.text.diff('Hello\n', 'Helli\n'))
+          async:send(vim._diff('Hello\n', 'Helli\n'))
         end
         local on_async = function(ret)
           vim.rpcnotify(1, 'result', ret)
@@ -372,7 +372,7 @@ describe('threadpool', function()
     it('work', function()
       exec_lua [[
         local work_fn = function()
-          return vim.text.diff('Hello\n', 'Helli\n')
+          return vim._diff('Hello\n', 'Helli\n')
         end
         local after_work_fn = function(ret)
           vim.rpcnotify(1, 'result', ret)