test: make the stress test slightly less stressful on slower machines

Without acceleration this part of the test takes over 10 minutes (!),
which feels quite unnecessary. Let's cut down the number of stuff we
dump to the journal in such case, but keep the original value if we run
with acceleration (since in that case it takes less than 10 seconds).

(cherry picked from commit ff40235b9b)

test/units/testsuite-04.sh

@@ -8,7 +8,8 @@ trap "journalctl --rotate --vacuum-size=16M" EXIT
 
 # Rotation/flush test, see https://github.com/systemd/systemd/issues/19895
 journalctl --relinquish-var
-for _ in {0..50}; do
+[[ "$(systemd-detect-virt -v)" == "qemu" ]] && ITERATIONS=10 || ITERATIONS=50
+for ((i = 0; i < ITERATIONS; i++)); do
     dd if=/dev/urandom bs=1M count=1 | base64 | systemd-cat
 done
 journalctl --rotate