test: don't store udev worker coredumps in journal

udev before #30532 may kill the worker process together with a slow
program, and when running with sanitizers the resulting coredump might
be too big to fit into journal (or the space currently available for
journal):

[   30.086194] systemd-journald[330]: Failed to write entry to /var/log/journal/e87de9ccbacf4b88924ff6d9ecaaa82d/system.journal (50 items, 68326399 bytes) despite vacuuming, ignoring: Argument list too long

This then makes the test fail, as it checks for the presence of the
coredump.

Since we don't really care about the coredump in this specific case (as
it is an expected one), let's just temporarily override the
testsuite-wide Storage=journal and store the coredumps externally.

This is a v255-stable-only patch, since after #30532 the test no longer
checks for coredumps.

(cherry picked from commit 27fc7a0321)

test/units/testsuite-17.03.sh

@@ -8,6 +8,9 @@ KILL_PID=
 setup() {
     mkdir -p "${TEST_RULE%/*}"
     [[ -e /etc/udev/udev.conf ]] && cp -f /etc/udev/udev.conf /etc/udev/udev.conf.bak
+    # Don't bother storing the coredumps in journal for this particular test
+    mkdir -p /run/systemd/coredump.conf.d/
+    echo -ne "[Coredump]\nStorage=external\n" >/run/systemd/coredump.conf.d/99-storage-journal.conf
 
     cat >"${TEST_RULE}" <<EOF
 ACTION=="add", SUBSYSTEM=="mem", KERNEL=="null", OPTIONS="log_level=debug"
@@ -32,6 +35,7 @@ teardown() {
     rm -rf "$TMPDIR"
     rm -f "$TEST_RULE"
     [[ -e /etc/udev/udev.conf.bak ]] && mv -f /etc/udev/udev.conf.bak /etc/udev/udev.conf
+    rm /run/systemd/coredump.conf.d/99-storage-journal.conf
     systemctl restart systemd-udevd.service
 }