gqltest: add test for external code host links

Created by: unknwon

E2e test:

https://github.com/sourcegraph/sourcegraph/blob/b09c208ad6ab17227a10614d997656b8b3ae826c/client/web/src/end-to-end/end-to-end.test.ts#L1096-L1119

GraphQL query:

https://github.com/sourcegraph/sourcegraph/blob/e996e6a4d0351f25289900680dbc63e7e6c77dfd/client/web/src/repo/backend.ts#L229-L241

Ref: https://github.com/sourcegraph/sourcegraph/issues/16135