authz: add integration test for GitHub permissions

Created by: unknwon

Copy from comment:

// This integration test performs a repository-centric permissions syncing against
// https://github.com, then check if permissions are correctly granted for the test
// user "sourcegraph-vcr-bob", who is a outside collaborator of the repository
// "sourcegraph-vcr-repos/private-org-repo-1".

In addition, this PR moves part of enterprise/cmd/frontend/internal/authz to internal/authz as they're no longer only related to frontend anymore.

Please review by commit.

Fixes #11614