test/crimson: mark final class "final"

test/crimson: mark final class "final"

silences warning from Clang like:

../src/test/crimson/seastore/test_object_data_handler.cc:33:16: warning: class with destructor marked 'final' cannot be inherited from [-Wfinal-dtor-non-final-class]
  ~TestOnode() final = default;
               ^
../src/test/crimson/seastore/test_object_data_handler.cc:20:7: note: mark 'TestOnode' as 'final' to silence this warning
class TestOnode : public Onode {
      ^
1 warning generated.

Signed-off-by: Kefu Chai <kchai@redhat.com>