fix/ert-multiline-explanation

[Phillip Lord]

I would appreciate feedback on whether the change on
fix/ert-multiline-explanation would be a good one.

At the moment, ert allows you to attach explanation functions to explain
why tests have failed. Unfortunately, these explanations are printed out
using "pp" which escapes new lines. So multiline explanations are,
largely, unreadable.

So this patch ignores the value of pp-escape-newlines and resets it to
nil for the duration.