tests: fix negative asserts with `grep` not working

Using `grep -v` without `-z` will return 0 even if there is a match
found as all the non-matching lines will be matched. Instead of using
`grep -vqz`, `(! grep ...)` is more readable.

The brackets are necessary as `! grep` will not trigger `set -e`[0], so we
run it inside a subshell to use its non-zero exit code.

[0]: https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#The-Set-Builtin

0 files changed, 0 insertions, 0 deletions