silkworm_infra_test: slow protocol stub tests on macOS

[battlmonstr]

Run:

time ./build/silkworm/infra/silkworm_infra_test --min-duration=1

Reports 4 tests that take almost exactly 5 sec each:

5.008 s: ETHBACKEND protocol version with server stub
5.007 s: KV protocol version with server stub
5.004 s: MINING protocol version with server stub
5.008 s: TXPOOL protocol version with server stub
0.57s user 0.29s system 3% cpu 21.823 total

This is a sign of "sleep"-driven tests, and should be fixed.
This can be a 20 sec win for make test.