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/zonechecks/tests.sh

@@ -16,7 +16,7 @@ set -e
 . ../conf.sh
 
 DIGOPTS="-p ${PORT}"
-RNDCCMD="$RNDC -c ../common/rndc.conf -p ${CONTROLPORT} -s"
+RNDCCMD="$RNDC -c ../_common/rndc.conf -p ${CONTROLPORT} -s"
 
 status=0