The hostname tests reimplemented the generation algorithm inline, so
they kept passing if the real code drifted; check the actual
generate_hostname path instead, and drop the tautological
check_user_namespace test.
Extract utils::random_suffix and replace the misleading
HOSTNAME_SUFFIX_BITS constant (it held the 2^24 entropy space, not a
bit count, and the suffix length was derived through a log2()/4
round-trip) with HOSTNAME_SUFFIX_LEN. The QEMU VM hostname generator
duplicated the algorithm with different mixing; both callers now share
the helper.