no untyped tests

[Illviljan]

• Closes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[max-sixty]

I'm don't think disallow_untyped_defs within tests gives us much, and adds a tax of -> None everywhere, especially for folks who aren't used to this. Tests rarely return anything, so having typed functions isn't that helpful.

check_untyped_defs is the big win, because then tests are used to check types. So any work to move modules into that list would be v helpful.

(IIRC disallow_untyped_defs was the only way of having all types checked before check_untyped_defs became available. So that was my original approach to the problem, but is now rendered obsolete..)

[Illviljan]

Yeah, I thought we always forced at least -> None nowadays.
I thought it was a good way to spot tests that weren't tested and taught users to ponder about what the function inputs and outputs.

Considering all the tricky ignores related to DataTree I find using check_untyped_defs too magical and far from default behavior, and here's the debug PR to figure it out.