test: Duplicate fields in queries with fragments

[ajeetdsouza]

GRAPHQL-654 has been fixed in #6596, this PR simply adds a test for it. The problem occurring was that using multiple fragments containing the same field would cause that field to appear multiple times in the output. Example:

Query:

fragment TaskID on Task {
  remoteId: id
  remoteType: __typename
}

fragment Task on Task {
  title
  remoteId: id
  completed
}

query {
  queryTask {
    ...TaskID
    ...Task
  }
}

Schema:

type Task {
  id: ID!
  title: String!
  completed: Boolean!
}

The resultant JSON listed remoteId twice, which was a bug.

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