types: remove dependency on extsvc clients

Created by: bobheadxi

Another way to resolve the import cycle in https://github.com/sourcegraph/sourcegraph/pull/28117 , instead of the reverted https://github.com/sourcegraph/sourcegraph/pull/28149.

The import cycle is caused by types importing various extsvc packages that each have clients to their respective external services, causing an import of httpcli. However, these types are only used in testing - this moves those test types out into types/typestest (mirroring standard library conventions like http/httptest). Diff only affects tests.