Unify Gremlin testing behind Gherkin

We currently use Gherkin for testing GLVs, translators and the grammar. The final step is to get Gherkin tests acting as the primary tests in the Gremlin Test Suite itself so that they may be run on the JVM for providers. Initially, at least in 3.6.0, it makes sense to maintain the old tests where they are but the ultimate goal should be to deprecate and remove the process suite and make the structure suite optional. In this way we aren't too opinionated about how providers implement their graph and are only interested in enforcing Gremlin semantics

Given improvements in null handling on TINKERPOP-2605 a lot of new gherkin tests are arriving. Need to add null annotations for graphs that can store null and include testing for such cases. This might be a separate issue, but for tying it to this one for now.