[9.18] fix: test: "nextpart" piped to "grep -q" doesn't work as expected

`nextpart file | grep -q` doesn't work as expected.  `grep -q` is not
required to read all of the input and that causes `nextpart` to fail.

Closes #5566

Backport of MR !11090

Merge branch 'backport-5566-nextpart-piped-to-grep-q-doesn-t-work-as-expected-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11093