Rename $HOSTNAME to $HOST_NAME to silence checkbashisms

Rename $HOSTNAME to $HOST_NAME to silence checkbashisms

checkbashisms warns about possible reliance on HOSTNAME environmental
variable which Bash sets to the name of the current host, and some
commands may leverage it:

    possible bashism in builtin/tests.sh line 199 ($HOST(TYPE|NAME)):
    grep "^\"$HOSTNAME\"$" dig.out.ns1.$n > /dev/null || ret=1
    possible bashism in builtin/tests.sh line 221 ($HOST(TYPE|NAME)):
    grep "^\"$HOSTNAME\"$" dig.out.ns2.$n > /dev/null || ret=1
    possible bashism in builtin/tests.sh line 228 ($HOST(TYPE|NAME)):
    grep "^; NSID: .* (\"$HOSTNAME\")$" dig.out.ns2.$n > /dev/null || ret=1

We don't use the variable this way but rename it to HOST_NAME to silence
the tool.