kmemdup() is easier than kmalloc() + memcpy(), per lkp bot.

Also make the input `suite` as const since we're now always making
copies after commit a127b154 ("kunit: tool: allow filtering test
cases via glob").
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NDaniel Latypov <dlatypov@google.com>
Reviewed-by: NDavid Gow <davidgow@google.com>
Reviewed-by: NBrendan Higgins <brendanhiggins@google.com>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>