pallet-staking: Put tests behind `cfg(debug_assertions)` #953

Job	Run time
set-image	10s
check-runtime-migration (asset-hub-westend)	6m 2s
check-runtime-migration (asset-hub-rococo)	29m 40s
check-runtime-migration (bridge-hub-westend)	4m 19s
check-runtime-migration (bridge-hub-rococo)	4m 26s
check-runtime-migration (contracts-rococo)	8m 17s
check-runtime-migration (collectives-westend)	4m 13s
check-runtime-migration (coretime-rococo)	4m 24s
	1h 1m 31s

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