Some tests don't have a mock root server configured, because they don't
need one. However, these tests might still leak queries to actual name
servers. Add a shared root hints file which can serve as a blackhole for
these queries.
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.
