Rename system test directory with common files to _common

The old name "common" clashes with the convention of system test
directory naming. It appears as a system test directory, but it only
contains helper files.

To reduce confusion and to allow automatic detection of issues with
possibly missing test files, rename the helper directory to "_common".
The leading underscore indicates the directory is different and the its
name can no longer be confused with regular system test directories.

bin/tests/system/_common/rndc.conf

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0.  If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+options {
+	default-key     "rndc_key";
+};
+
+key rndc_key {
+	algorithm       hmac-sha256;
+	secret          "1234abcd8765";
+};