Network-related unit tests became unstable after merging the loopmgr branch

Merging !6040 (merged) caused the failure rate for network-related unit tests to become significantly higher on the main branch. The frequently-failing tests include:

• doh_test
• tcp_test
• tls_test - issues #3594 (closed) and #3597 (closed)
• tlsdns_test
• udp_test - fixed in !6888 (merged) and !6889 (merged), and there are cases when the udp_test just gets stuck and times out

Sample failures from a single pipeline:

• https://gitlab.isc.org/isc-private/bind9/-/jobs/2819710 (tcp_test)
• https://gitlab.isc.org/isc-private/bind9/-/jobs/2819711 (doh_test)
• https://gitlab.isc.org/isc-private/bind9/-/jobs/2819748 (tcp_test)

These failures should be addressed sooner rather than later as they wreak havoc in CI results for main. Leaving them unfixed might prevent us from noticing newly-introduced bugs in a timely manner.