expand bitswap test suite

[dignifiedquire]

• [ ] multiple providers (all have the content)
  • make sure cancels are sent out
• [ ] multiple providers (some are not dialable)
  • make sure content is fetched & failed dials are cleaned up
• [ ] no dialable providers
  • make sure the request times out
• [ ] benchmarks
  • [ ] 1-1 transfer on the same machine, (msg sizes: 128, 256, 4KiB, 16KiB, 512KiB 1MiB, 2MiB)