check_equal(::NamedTuple, ::NamedTuple) doesn't check names

e.g. 
```julia
julia> check_equal((;a=2, b=2), (;a=2,c=2))
Test Passed
```

This is because it hits the general iterator fallback.


This isn't super high priority, since it only comes up in chaecking `add!!` behavour
(as neither value is a `AbstractTangent` subtype).

Still would be nice to fix.
Need to channel it towards one of our overloads for structured types
