Group resolution tests into submodules

[alexcrocha]

Part of #649

By grouping the tests into semantically coherent submodules, we get collapsible navigation in editors and a layout that shows the resolver's surface area: constant lookup, ancestors/MRO, method dispatch, state & storage, dynamic/meta, and RBS. No test bodies change, each commit is a pure relocation.

Final layout:

Constant Lookup:

mod constant_resolution_tests;
mod constant_alias_tests;

Ancestors & Method Resolution Order:

mod superclass_tests;
mod include_tests;
mod prepend_tests;
mod mixin_dedup_tests;
mod object_ancestors_tests;
mod singleton_ancestors_tests;

Method Dispatch:

mod method_tests;
mod method_alias_tests;

State & Storage:

mod variable_tests;
mod declaration_creation_tests;
mod singleton_class_tests;
mod fqn_and_naming_tests;
mod todo_tests;

Dynamic / Meta:

mod dynamic_namespace_tests;
mod promotability_tests;

RBS:

mod rbs_tests;

[alexcrocha]

• Group resolution tests into submodules #756 👈 (View in Graphite)
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[vinistock]

IMO, I think the modules are bit too granular. For example, I would have a single ancestors module for everything related to ancestor chains rather than include, prepend... Especially because some of the scenarios mix things, so where would those go?

That said, we can always adjust