Re-enable contract test in CI

[alxiong]

afaiu, our nitro fork (branch v3.6.7-merge) is tracking nitro's v3.6.7 whose contract/ is tracking bdb8f8. Since the CI is passing on that branch, we should re-enable the contract test in the CI:

https://github.com/OffchainLabs/nitro-contracts/blob/bdb8f8c68b2229fe9309fe9c03b37017abd1a2cd/.github/workflows/contract-tests.yml

(@Sneh1999 disable them in e38dd0c)