test(e2e): fix race in 'proposer invalidates multiple checkpoints'

yarn-project/end-to-end/src/e2e_epochs/epochs_invalidate_block.parallel.test.ts

@@ -378,12 +378,19 @@ describe('e2e_epochs/epochs_invalidate_block', () => {
     // Wait for at least one checkpoint to be mined so that any in-progress slot has completed
     const initialCheckpointNumber = (await nodes[0].getChainTips()).checkpointed.checkpoint.number;
     await test.waitUntilCheckpointNumber(CheckpointNumber(initialCheckpointNumber + 1), test.L2_SLOT_DURATION_IN_S * 4);
+
+    // Align to the start of an L2 slot before computing the bad slots, so we have a generous
+    // buffer to push the malicious config to badSlot1's proposer before it snapshots its config
+    // into a new CheckpointProposalJob. Under proposer pipelining, that job is built during the
+    // last L1 slot of the previous L2 slot (when getEpochAndSlotInNextL1Slot first returns the
+    // proposer's target slot), so the practical window is somewhat less than a full L2 slot.
+    await test.monitor.waitUntilNextL2Slot();
     const { l2SlotNumber: currentSlot } = await test.monitor.run();
     logger.warn(`First checkpoint mined, current slot is ${currentSlot}`);
 
-    // Pick the next two slots after the current one, with a 1-slot gap to account for pipelining
-    const badSlot1 = SlotNumber.add(currentSlot, 2);
-    const badSlot2 = SlotNumber.add(currentSlot, 3);
+    // Pick the next two slots with a 2-slot gap to account for pipelining plus a margin
+    const badSlot1 = SlotNumber.add(currentSlot, 3);
+    const badSlot2 = SlotNumber.add(currentSlot, 4);
     const badSlots = [badSlot1, badSlot2];
     const badProposers = await Promise.all(badSlots.map(s => test.epochCache.getProposerAttesterAddressInSlot(s)));