Notes on the state of the tests

[FauxFaux]

The tests aren't a great experience for someone downloading this codebase.

This commit adds:

• a note to the README about what the errors mean
• an ignore on a test that hangs forever on purpose

It doesn't fix two outstanding tests, which attempt to read a file named dumps/ip_link.bin, which appears to have not been committed? Could that please be rectified, or those tests removed?

[FauxFaux]

The failing travis is a bug in pnet, not due to my changes. I'm doing a (different) PR for them anyway.

[bkchr]

Ty