Defensive traits: Ensure doc tests are also working without `debug_assertions` (#12952)

* Defensive traits: Ensure doc tests are also working without `debug_assertions`

Closes: https://github.com/paritytech/substrate/issues/12940

* Only run 'panics' tests if they will panic

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>