This test checks a diagnostic for invalid use of internal packages.

This list error changed in Go 1.21.

See TestValidImportCheck_GoPackagesDriver for a test that no diagnostic
is produced when using a GOPACKAGESDRIVER (such as for Bazel).

-- flags --
-min_go=go1.21

-- go.mod --
module bad.test

go 1.18

-- assign/internal/secret/secret.go --
package secret

func Hello() {}

-- bad/bad.go --
package bad

import _ "bad.test/assign/internal/secret" //@diag("\"bad.test/assign/internal/secret\"", re"could not import bad.test/assign/internal/secret \\(invalid use of internal package \"bad.test/assign/internal/secret\"\\)"),diag("_", re"use of internal package bad.test/assign/internal/secret not allowed")