Refactor `Queryable` specs
Refactor `Queryable` specs

https://github.com/ruby-rdf/rdf-spec/pull/33 created some duplicate
tests; this refactors by moving them into a shared example set.

Additionally, it skips specs for `#query_pattern` and `#query_execute`
if they are not defined by the `Queryable`. This allows implementations
to fulfill the `#query` interface directly, if desired.

ruby-rdf/rdf-spec