[feature]: Add integration test for concurrent transfers with delayed block mining

[ffranr]

This issue stems from this comment by @jharveyb : #1068 (comment)

It would be beneficial to add an integration test that simulates multiple transfers running concurrently. This can be achieved by delaying the mining of a block. The test should submit a first transfer, then a second one, ensuring both are pending confirmation until the block is mined. Once the block is mined, both transfers should proceed as expected. This test would help verify the correct handling of concurrent transfers under these conditions.

[ffranr]

Might also be a good opportunity to refactor this itest: #1074 (comment)

[jharveyb]

Aye - and once the proof resume on restart work is all in I think we should be able to restart before that conf comes in and resume all parcels correctly if the conf happens while we are shut down.