Docs: add some example log lines that indicate a lack of memory when running the test suite

This may seem obvious when you know, but I spent one hour on the second one, thinking that "/tmp" was a "tmpfs" with a fixed size and trying to increase it somehow... Thankfully @crosbymichael found the problem immediately on IRC and advised I give more RAM to my VM.

I think having that kind of help in the doc will help newcomers to focus on more interesting problems.

Thanks