selftest, ptrace: Add selftest for syscall user dispatch config api

Validate that the following new ptrace requests work as expected

 * PTRACE_GET_SYSCALL_USER_DISPATCH_CONFIG
   returns the contents of task->syscall_dispatch

 * PTRACE_SET_SYSCALL_USER_DISPATCH_CONFIG
   sets the contents of task->syscall_dispatch

Signed-off-by: Gregory Price <gregory.price@memverge.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230407171834.3558-5-gregory.price@memverge.com