tests: improve actvititypub integration test code

- Make use of `test.MockVariableValue` to override variables for the
duration of the test.
- Don't needlessly call `onGiteaRun`, its only needed when a HTTP server
needs to be called by the code.
- When `onGiteaRun` is used, make use of the passed parameters, such as
the passed `*testing.T` variable and `*url.URL` (this also avoids
needing to serve the routers in the test code again).
- Use `(*url.URL).JoinPath` to craft new URLs.
- Don't override `setting.AppURL` & `setting.Database.LogSQL` when its
does not affect the test.
- Add empty fixture files for `FederatedUser` & `FederationHost` so they
are truncated and do not persist between tests.

models/git/main_test.go

@@ -11,6 +11,7 @@ import (
 	_ "code.gitea.io/gitea/models"
 	_ "code.gitea.io/gitea/models/actions"
 	_ "code.gitea.io/gitea/models/activities"
+	_ "code.gitea.io/gitea/models/forgefed"
 )
 
 func TestMain(m *testing.M) {