test: store coredumps in journal

To make debugging much easier, especially for crashes in tests under
QEMU, let's store the entire coredump bundle in the systemd journal,
which is usually kept around by various CIs. Right now, we usually end
up with a journal, but without the coredump itself, which is pretty
useless.

test/test-functions

@@ -516,6 +516,8 @@ install_systemd() {
 
     # enable debug logging in PID1
     echo LogLevel=debug >> $initdir/etc/systemd/system.conf
+    # store coredumps in journal
+    echo Storage=journal >> $initdir/etc/systemd/coredump.conf
 }
 
 get_ldpath() {