File-scoped Go tests compiled a synthetic package from the one file, so helpers in sibling files, TestMain and the external _test package were missing ("undefined: helper"). They now run in their real package with tests declared in sibling files excluded from the -run selection, in Run, Debug and reruns; build-file selection is kept, and a name shared by the internal and external test packages gives an actionable error instead of running the wrong test.
Co-authored-by: shohruh-genius <[email protected]>