Unit test gql cache functions

[Ullaakut]

• getCache(ctx *context.Context, req *http.Request, pagination string) (*http.Response, error)
• readCachedResponse(filename string, req *http.Request) (*http.Response, error)
• putCache(ctx *context.Context, req *http.Request, pagination string, body []byte) error
• cacheEntryFilename(ctx *context.Context, url string) string
• listFilePagination(cursor string) string
• contribFilePagination(cursor string, year int) string