`v.id` -> `v.id()` in a gremlin string in a test

`v.id` was working because `id` is a public field on TinkerGraph
vertices, but that field isn't part of the [Vertex][1] interface. The
id() method should work regardless of vendor. (The id field isn't
available on DSE).

[1]: http://tinkerpop.apache.org/javadocs/3.1.5/core/index.html?org/apache/tinkerpop/gremlin/structure/Vertex.html