Ensure that CheriBSD testsuite runs drop to debugger on panic

This is currently the default behaviour but it may change in the future.
Explicitly set the sysctl to ensure we get the expected output.

pycheribuild/boot_cheribsd/__init__.py

@@ -974,6 +974,8 @@ def _do_test_setup(qemu: QemuCheriBSDInstance, args: argparse.Namespace, test_ar
                    test_setup_function: "Optional[Callable[[CheriBSDInstance, argparse.Namespace], None]]" = None):
     smb_dirs = qemu.smb_dirs
     setup_tests_starttime = datetime.datetime.now()
+    # Print a backtrace and drop into the debugger on panic
+    qemu.run("sysctl debug.debugger_on_panic=1; sysctl debug.trace_on_panic=1")
     # Enable userspace CHERI exception logging to aid debugging
     qemu.run("sysctl machdep.log_user_cheri_exceptions=1 || sysctl machdep.log_cheri_exceptions=1")
     if args.enable_coredumps: