Include prepare-softhsm2.sh in source tarballs

Include prepare-softhsm2.sh in source tarballs

The util/prepare-softhsm2.sh script is useful for initializing a working
SoftHSM environment which can be used by unit tests and system tests.
However, since it is a test-specific script, it does not really belong
in the util/ subdirectory which is mostly pruned during the BIND source
tarball creation process.  Move the prepare-softhsm2.sh script to
bin/tests/ so that its location is more appropriate for its purpose and
also so that it does not get removed during the BIND source tarball
creation process, allowing it to be used for setting up test
environments for tarball-based builds.