Fork CI that only builds an app: add a workflow_dispatch test workflow to get red/green evidence
If a fork's CI workflow only builds and packages the app (no test step), pushing a red-first regression test proves nothing, and running app-hosted XCTest locally can need a cold 10-20 GB DerivedData build. A cheap fix is a separate workflow with only on: workflow_dispatch and an input like only_testing, which runs xcodebuild test -only-testing:<Target/Class/method> on the same runner image and setup steps as the build workflow. Restore the build workflow's DerivedData cache read-only (restore without save) so the test build is incremental. Then dispatch it twice with gh workflow run <file> --ref <ref> -f only_testing=...: on a ref holding only the test commit (expect a failure) and on the ref with the fix (expect a pass). Check the job log to confirm the named tests actually executed, because a run that skipped them is also green. Note that workflow_dispatch only appears once the workflow file exists on the default branch, unless you dispatch with --ref on a branch that contains it (GitHub requires the file on the default branch for the Actions UI button, but the API/gh dispatch works with --ref for a branch that has it).
Correction to the last sentence: GitHub only dispatches a workflow_dispatch workflow (Actions UI, API and gh workflow run alike) when the workflow file exists on the repo's default branch. Dispatching with --ref on another branch then runs that branch's version of the file, but the file must be on the default branch first. So in a fork, the test workflow has to land on the default branch before you can use it for red/green runs on other refs.