This reports the use of standard library symbols that are too new
for the Go version in force in the referring file,
as determined by `go` directive in `go.mod` and build tags on the file.

	{GOOS: "openbsd", GOARCH: "amd64", CgoEnabled: true},
	{GOOS: "openbsd", GOARCH: "amd64"},
}

func contextName(c *build.Context) string {
	s := c.GOOS + "-" + c.GOARCH
	if c.CgoEnabled {
		s += "-cgo"
	}
	if c.Dir != "" {
		s += fmt.Sprintf(" [%s]", c.Dir)
	}
	return s
}

var internalPkg = regexp.MustCompile(`(^|/)internal($|/)`)

var exitCode = 0

func Check(t *testing.T) {