kunit: use kmemdup in kunit_filter_tests(), take suite as const

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: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>