[test] Fix assertion on arrays of BigNumbers

[nxqbao]

The hardhat-deploy package uses higher version of ethers which causes the following test case failed.

console.log(await validatorContract.getJailUntils(expectingValidatorSet));
console.log(BigNumber.from(blockNumber).add(jailDurationForUnavailabilityTier2Threshold));

Before upgrade

[ BigNumber { value: "57658" } ]
BigNumber { value: "57658" }

After upgrade

[ BigNumber { _hex: '0xe13a', _isBigNumber: true } ]
BigNumber { value: "57658" }