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/fetchlimit/tests.sh

@@ -18,7 +18,7 @@ set -e
 DIGCMD="$DIG @10.53.0.3 -p ${PORT} +tcp +tries=1 +time=1"
 
 rndccmd() (
-    "$RNDC" -c ../common/rndc.conf -p "${CONTROLPORT}" -s "$@"
+    "$RNDC" -c ../_common/rndc.conf -p "${CONTROLPORT}" -s "$@"
 )
 
 burst() {