Testing: validate our GraphQL queries against GHES schema - #3689

To ensure we don't generate GraphQL queries that aren't compatible with an older version of the GitHub API (GHES), we'd like to come up with a mechanism to validate each GraphQL query that we generate in tests against a GraphQL schema.

WIP:

$ make ghes-2.21.graphql
$ go run ./cmd/gql-validate ghes-2.21.graphql '{viewer}'
query error: input:1: Field "viewer" of type "User!" must have a selection of subfields. Did you mean "viewer { ... }"?

exit status 1

TODO:

• Hook into our test suite (probably httpmock.GraphQL calls) and verify each GraphQL query against the schema of the oldest supported Enterprise version
• Figure out how to handle queries that we dynamically generate based on schema instrospection

Ref. #3688 #2996 #2035