If you run tests through the loopback interface, you should be able to use any of the 127.0.0.0/8 addresses, instead of 127.0.0.1 (a.k.a ”localhost”). Each address has its independent port space, so you would avoid collisions/contention.
Unix sockets skip all of tcp though, so I'd recommend them vs loopback, if possible. I hear Linux short circuits a lot of tcp for loopback, but there's probably still a bunch of connection state that doesn't need to be there. FreeBSD runs normal tcp on loopback, and you can get packet loss if you overrun the buffer, and congestion collapse and the whole nine yards. Great for validating the tcp stack, not the best for performance; better to skip it.
This discussion [1] sheds some light on the matter, but does not fully explain the reasons for differing behaviour across OS's.
[1]: https://serverfault.com/questions/293874/why-cant-i-ping-an-...
# ping -c 1 10.10.10.10
1 packets transmitted, 0 received
# ip addr add 10.10.10.1/24 dev lo
# ping -c 1 10.10.10.10
64 bytes from 10.10.10.10: time=0.023 ms
1 packets transmitted, 1 receivedAs opposed to a normal interface where only the specific address is a local address, but the rest of the network specified are accessible through that interface.
Maybe one /8 wasn't enough addresses for you, so you added more? Doesn't seem like an unreasonable way to behave, even if it's different than BSD behavior; it certainly makes it easier to use lots of loopback addresses.