auto merge of #1672 : kmcallister/servo/tests, r=pcwalton

I'd like to have an `iframe` reftest as well but `iframe`s are currently broken.  Will look into that next.
Build libstd etc. with debug info when Servo configured with --enable-debug
Change the mechanism behind summit-crash.html
Move FailureMsg contents into a cloneable struct
Add a helper for sending a message on task failure
wip
Restore hard-fail mode
Use hard-fail for reftests