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>

test/units/TEST-59-RELOADING-RESTART.sh

@@ -174,6 +174,6 @@ systemctl daemon-reload
 systemctl start testservice-fail-restart-debug-59.service
 wait_on_state_or_fail "testservice-fail-restart-debug-59.service" "failed" "15"
 journalctl --sync
-journalctl -b | grep -q "Failed to follow symlinks on /nonexistent-debug-59: No such file or directory"
+journalctl -b | grep "Failed to follow symlinks on /nonexistent-debug-59: No such file or directory" >/dev/null
 
 touch /testok