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/resolver/tests.sh

@@ -21,7 +21,7 @@ dig_with_opts() {
 }
 
 rndccmd() {
-	"${RNDC}" -c ../common/rndc.conf -p "${CONTROLPORT}" -s "${@}"
+	"${RNDC}" -c ../_common/rndc.conf -p "${CONTROLPORT}" -s "${@}"
 }
 
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