Enable larger joins for `p2p` in `test_join.py`

[hendrikmakait]

Now that #633 has been merged, we can test larger joins (https://github.com/coiled/coiled-runtime/blob/2e0f4a6f395868acf20db960588b64d4d1f8c80a/tests/benchmarks/test_join.py#L12-L17)