tests: Replace "localhost" domain name with the equivalent IPv4 address

[eskultety]

It's common that "localhost" translates with both IPv4 and IPv6. The simple containerized test PyPI server we use in our CI doesn't listen on IPv6 which may cause unpredictable issues on hosts where IPv6 is enabled and IPv6 would get prioritized by the low level network stack connection primitives. Therefore, use an IP explicitly to avoid any potential issues during pytest run, e.g.:

DEBUG Starting new HTTP connection (1): localhost:8080
DEBUG ('Connection aborted.',
       ConnectionResetError(104, 'Connection reset by peer'))

PR depends on: cachito-testing/cachi2-pip-custom-index#1

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[ben-alkov]

1. Did you mean to use your clone of cachito-testing-cachi2-pip-custom-index?
2. Should we make the local v4 IP a global (instead of hardcoding everywhere)?

Otherwise, LGTM

[eskultety]

2. Should we make the local v4 IP a global (instead of hardcoding everywhere)?

I can create a global variable for that if desired.

[eskultety]

Integration tests for pip passed (the failures are unrelated), so I'm going to merge cachito-testing/cachi2-pip-custom-index@e3916a8, update this one and hopefully merge so that no other PRs are disrupted by the change in tests.