pallet-staking: Put tests behind cfg(debug_assertions) (#4620)

Otherwise these tests are failing if you don't run with
`debug_assertions` enabled, which happens if you run tests locally in
release mode.

substrate/frame/staking/src/tests.rs

@@ -5251,6 +5251,7 @@ mod election_data_provider {
 	// maybe_max_len`.
 	#[test]
 	#[should_panic]
+	#[cfg(debug_assertions)]
 	fn only_iterates_max_2_times_max_allowed_len() {
 		ExtBuilder::default()
 			.nominate(false)
@@ -5939,6 +5940,7 @@ fn min_commission_works() {
 
 #[test]
 #[should_panic]
+#[cfg(debug_assertions)]
 fn change_of_absolute_max_nominations() {
 	use frame_election_provider_support::ElectionDataProvider;
 	ExtBuilder::default()