Move `auto_update_tests.py` into scripts directory. NFC

[sbc100]

So it lives alongside update_lit_checks.py and update_help_checks.py

[kripken]

Hmm, makes sense, but then maybe check.py also makes sense to move - then we'd have no scripts in the top level.

The current state before this PR also makes sense to me, of having just the 2 main scripts there: check and update (and all their helpers are in subdirs).

But I don't feel strongly here.

[sbc100]

I think its ok to keep check.py at the top level for now.. but I also don't feel strongly. @tlively what do you think? Ok to land this as is or should I move check too?

[tlively]

I don't feel too strongly. I think it would be fine to move auto_update_tests.py to scripts/ and keep check.py where it is. It would be great to document auto_update_tests.py in the README, though. Now that it's not in the top directory, it's less discoverable.

[sbc100]

Updated the README to point to the two different auto-update scripts.

[sbc100]

OK to land this now, with the documentations changes?