devnet: devnet-test and CI improvements

[protolambda]

General improvement for a faster devnet CI job.
This reduces the devnet time from ~16-18 min to ~6-7 min. (note: some of the docker build improvements are used already, and improving more in separate PRs like #7811 )

Changes:

• Set devnet recommit time to 100ms, so tx-inclusion is not delayed by a block
• Avoid docker-build: we already load the images from the workspace, built by the previous job. We can run the devnet with those.
• Change the devnet python script to run the hardhat tests in parallel: this required adding an extra test option to select the signer, and hardhat configuration to support more devnet signers, to avoid conflicts between tests (running on the same network, thus need different accounts to have no nonce-increment race conditions).
• Increase the devnet L2 output submission interval from 6 L2 blocks to 10 L2 blocks. This makes the proposer fall behind less (it needs to wait for batcher and confirm txs already, which takes more than 6 seconds), and by not falling behind the test is faster. We might want to bump this number up higher later.

[protolambda]

Re-added to merge queue. The merge-queue job failed, and wasn't letting the next PR go through, stuck on CI status report. It flaked on 4 cannon tests in op-e2e.