op-e2e: fix p2p gossip flakiness

[protolambda]

Small test setup fixes:

• Disable batcher to see L2 gossip effect in isolation. Adjusting the block time doesn't work well when it runs in a laggy environment.
• Increase timeouts.
• Only start sequencing after the verifiers are running, to give the verifiers a proper chance of receiving the first few payloads, instead of publishing those while the verifiers might not be fully up and running yet.

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[protolambda]

@trianglesphere I found that the verifiers were missing the first few payloads because the sequencer was too quick to publish them, before the verifiers were running. I changed it now to start sequencing only later into the test.