Fix duration saying never when it's unlimited

internal/config/config_test.go

@@ -78,6 +78,14 @@ func TestFormatRoundTrips(t *testing.T) {
 		if got, err := ParseDuration(FormatDuration(d)); err != nil || got != d {
 			t.Errorf("FormatDuration(%s) = %q, which parses back to %s (%v)", d, FormatDuration(d), got, err)
 		}
+		// FormatLifetime reaches the upload form as the expiry placeholder, so
+		// what it prints has to be something a caller could type back.
+		if got, err := ParseDuration(FormatLifetime(d)); err != nil || got != d {
+			t.Errorf("FormatLifetime(%s) = %q, which parses back to %s (%v)", d, FormatLifetime(d), got, err)
+		}
+	}
+	if got := FormatLifetime(Unlimited); got != "unlimited" {
+		t.Errorf("FormatLifetime(Unlimited) = %q, want %q", got, "unlimited")
 	}
 }