Fix parser bug with sequential selection sets

[ilmoraunio]

It was possible to write:

(oksa.core/gql [[:foo {} [:qux :baz]] [:basho]])
; or
(oksa.core/gql
  (oksa.alpha.api/select (oksa.alpha.api/field :foo
                                (oksa.alpha.api/select :qux :baz))
                         (oksa.alpha.api/select :basho)))

resulting in incorrect GraphQL:

{foo{qux baz}{basho}}

This PR fixes the parser to consider sequential selection sets as invalid. We also remove an API test case that was invalid to begin with.