fix: test: Make deleg cleanuptests memory assertions 32-bit-safe

Each address entry stored by dns_delegset_addaddr() is an
isc_netaddrlink_t, whose size depends on sizeof(void *) via the
ISC_LINK macro (24 bytes of address + two prev/next pointers): 40
bytes on 64-bit, 32 bytes on 32-bit. The hardcoded 4 MB / 8 MB
ranges only held on 64-bit, so dns_deleg_cleanuptests failed on
armv7l with isc_mem_inuse() returning ~3.2 MB.

Express the expected ranges in terms of sizeof(isc_netaddrlink_t)
so they scale with pointer width, and pull the 99999 entry count
out into a NENTRIES macro.

Close isc-projects/bind9#6012

Merge branch 'mnowak/armv7l-fix-dns_deleg_cleanuptests' into 'main'

See merge request isc-projects/bind9!12061