Enable text chat tests now that createOffer in Firefox doesn't randomly fail

Now that bug 896429 is fixed, we can enable the text chat tests again, as the intermittent fail should be fixed. I've run this various times locally, and it is now all passing where it was failing before.

Note that the patch is in tomorrow's (26th July) Firefox nightlies, so this is best merged after they are published (somewhere around noon - 2pm UTC).