Problem:
137f98cf64 added the `create` parameter to `tmpname()` but didn't
fully implement it.
Solution:
- Update impl for the `os.tmpname()` codepath.
- Inspect all usages of `tmpname()`, update various tests.
Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.
Closes https://github.com/neovim/neovim/issues/27004.
