Allow #![doc(test(attr(..)))] everywhere

[Urgau]

This PR adds the ability to specify #![doc(test(attr(..)))] at module level everywhere in addition to allowing it at crate-root.

This is motivated by a recent PR #140323 (by @tgross35) where we have to duplicate 2 attributes to every single f16 and f128 doctests, by allowing #![doc(test(attr(..)))] at module level (and everywhere else) we can omit them entirely and just have (in both module):

#![doc(test(attr(feature(cfg_target_has_reliable_f16_f128))))]
#![doc(test(attr(expect(internal_features))))]

Those new attributes are appended to the one found at crate-root or at a previous module. Those "global" attributes are compatible with merged doctests (they already were before).

Given the small addition that this is, I'm proposing to insta-stabilize it, but I can feature-gate it if preferred.

Best reviewed commit by commit.

r? @GuillaumeGomez