test(fs): get tmpdir robustly #23021

Problem:
helpers.tmpname() may create a local file, depending on circumstances.

Solution:
Only use helpers.tmpname() for its parent directory (the "temp root").
Use fs_mkdtemp() to actually get a unique name.

test/functional/core/fileio_spec.lua

@@ -19,7 +19,6 @@ local meths = helpers.meths
 local mkdir = helpers.mkdir
 local sleep = helpers.sleep
 local read_file = helpers.read_file
-local tmpname = helpers.tmpname
 local trim = helpers.trim
 local currentdir = helpers.funcs.getcwd
 local assert_alive = helpers.assert_alive
@@ -266,9 +265,7 @@ describe('tmpdir', function()
 
   before_each(function()
     -- Fake /tmp dir so that we can mess it up.
-    os_tmpdir = tmpname()
-    os.remove(os_tmpdir)
-    mkdir(os_tmpdir)
+    os_tmpdir = vim.loop.fs_mkdtemp(vim.fs.dirname(helpers.tmpname()) .. '/nvim_XXXXXXXXXX')
   end)
 
   after_each(function()