test: Stop using grep -q in integration test

When a TTY is attached to the test unit, grep -q will generate SIGPIPE
for the previous command in the pipeline which in combo with `pipefail`
will cause the command to fail with exit status 141 which will fail the
test.

Replace with >/dev/null to avoid this from happening.

See also https://www.gnu.org/software/grep/manual/html_node/Usage.html
> There is a related problem with Bash’s set -e -o pipefail. Since grep
> does not always read all its input, a command outputting to a pipe read
> by grep can fail when grep exits before reading all its input, and the
> command’s failure can cause Bash to exit.

Co-authored-by: Yu Watanabe <watanabe.yu+github@gmail.com>

docs/CODING_STYLE.md

@@ -999,3 +999,9 @@ SPDX-License-Identifier: LGPL-2.1-or-later
 
 - When modifying existing tests, please convert the test to use the new assertion
   macros from `tests.h` if it is not already using those.
+
+## Integration Tests
+
+- Never use `grep -q` in a pipeline, use `grep >/dev/null` instead. The former
+  will generate `SIGPIPE` for the previous command in the pipeline when it finds
+  a match which will cause the test to fail unexpectedly.