Ensure the allocator types in the string keyed tests have value_types matching the...

Ensure the allocator types in the string keyed tests have value_types matching the value type of the collection

Summary:
Because MSVC's standard library implementation is now checking that this is actually the case:
```
error C2338: set<T, Compare, Allocator> requires that Allocator's value_type match T (See N4659 26.2.1 [container.requirements.general]/16 allocator_type) Either fix the allocator value_type or define _ENFORCE_MATCHING_ALLOCATORS=0 to suppress this diagnostic.
```

I've gone with the "Fix the `value_type`" approach.

Reviewed By: yfeldblum

Differential Revision: D5441017

fbshipit-source-id: b30403d2e48f815177cac29c4e6eec4e79583660