Add note and workaround for nip.io in tests

[johnlettman]

This PR adds a note to the build documentation about the use of nip.io in tests. Networks implementing DNS rebinding protection may block the resolution of loopback addresses as documented on the nip.io page.

On my own network, this condition lead to failed tests.

The note includes a /etc/hosts workaround.

Checklist

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[balchua]

IMHO, this will be better have it documented in https://discuss.kubernetes.io/t/add-on-dashboard/11289, which can translate to the official MicroK8s documentation.

[johnlettman]

Hello @balchua !

I agree the thread you linked is the best place to outline the consideration of the nip.io domain and the changes of PR #2478.

In addition to the changes of that PR, the microk8s tests also use the nip.io domain (ingress mainly) and can fail as a result of the network using DNS rebinding protection. This PR targets the build documentation where testing information is located to warn the developer of this potential.

[ktsakalozos]

Thank you for this PR @jlettman just a minor change requests.

[johnlettman]

@ktsakalozos Might be nitpicking, but should the note be before the test procedure as a warning?
I wish GH flavored markdown incorporated the admonition feature akin to ASCIIDoc ;) maybe someday.

[neoaggelos]

Thanks @jlettman