add support for testing with graphql fragments

[punchcafe]

Motivation

This PR is to meet a need discovered recently while writing tens of tests using the same return types, and discovering that the GraphQLTestTemplate doesn't support querying with fragments (or at least not obviously):
For example, given the following graphqls query resource files:

• some-fragment.graphqls
• some-query-using-fragment.graphqls

graphQLTestTemplate.postForResource('graphql/some-query-using-fragment.graphqls') will fail.

Proposed Solution

This PR adds overloaded methods to GraphQLTestTemplate#postForResource and GraphQLTestTemplate#perform, so that the above scenario can be handled with:

graphQLTestTemplate.postForResource(
          'graphql/some-query-using-fragment.graphqls', 
          List.of(some-fragment.graphqls)
)

And can be applied to an arbitrary number of fragments.

[oliemansm]

Thanks for the contribution!