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

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

See merge request isc-projects/bind9!11090