fix: dev: Build the fuzzers without the libbindtest test library

Every fuzz target depended on libtest_dep, which forces building the
libbindtest shared library.  In a static build (as used by OSS-Fuzz)
that link fails: libbindtest's netmgr wrappers multiply-define symbols
that also live in the static libisc/libns archives, and the static
system libraries are not position independent.

Only fuzz_dns_qp actually uses the qp test helpers, so give it just
tests/libtest/qp.c via the new libtest_qp_dep and drop libtest_dep
from the fuzzers.

Assisted-by: Claude:claude-opus-4-8
Merge branch 'mnowak/fuzz-drop-libbindtest' into 'main'

See merge request isc-projects/bind9!12194