test: ✅ Fix race conditions in integration tests

[ffarall]

In this PR:

1. Fix race condition in skipTo waiting for proofs (eliminated sleeps).
2. Fix race condition where there could have been multiple PaymentStreamCharged events in debt-collection.test.ts.
3. Fix race condition where MSP didn't add a file to it's local File Storage in time in respond-multi-requests.test.ts.
4. Remove redundant iterations in waitForTxInPool.
5. Change file from smile.jpg to whatsup.jpg in bsp-threshold.test.ts to make it so that BSP_DOWN cannot immediately volunteer.
6. Simplify logic in bsp-threshold.test.ts and make it deterministic which BSP should be volunteering first (that shouldn't change given that their IDs and the file keys of the files uploaded don't change ever).
7. Fix race condition in bsp-threshold.test.ts where BSP was not yet synced with chain tip before waiting for volunteer.
8. Wait for BSPs to catch up to chain tip in submit-proofs.test.ts.
9. Add minimum check of number of txs in pool.
10. Fix race condition in storage-capacity.test.ts where a transaction was submitted with the BSP key, but the BSP node had already submitted a transaction with the same nonce.

[TDemeco]

LGTM 👍🏻