Tests leak goroutines because of go-libp2p and leveldb

[iand]

Migrated from libp2p/go-libp2p-kad-dht#906

We observed that the 32-bit test runner runs out of memory if we increase the test count to 100.

There are two areas where we leak goroutines after the test has stopped:

1. leveldb has a one-second delay before properly releasing all resources. There is nothing immediate we can do about that. If we added a delay of 1s + δ, all resources are released.
2. go-libp2p does not clean up fx resources properly. This is a known issue and will be fixed soon: libp2p.Host leaks goroutines libp2p/go-libp2p#2514

Revisit after libp2p/go-libp2p#2514 has been resolved.