Rollup merge of #156357 - RalfJung:slow-test, r=Mark-Simulacrum

tests: ip*_properties: avoid parsing the IP over and over again

This test somehow takes 12s to execute in Miri (on CI, i.e. on a slow machine), despite not having any loop. I suspect it's caused by us for some reason re-parsing the same string again and again. Let's just not do that.