Revert "test: don't panic on soft lockups"

Let's put this back in, as it could help with occasional machine lock ups
on overloaded systems (and it didn't help with the original issue
anyway).

This reverts commit 3a89904e45.

test/test-functions

@@ -571,6 +571,7 @@ run_qemu() {
     if ! get_bool "$INTERACTIVE_DEBUG" && ! get_bool "$TEST_SKIP_SHUTDOWN"; then
         kernel_params+=(
             "panic=1"
+            "softlockup_panic=1"
             "systemd.wants=end.service"
         )
     fi