Add TestExpectations file for Haiku

Just skip the only test that crashes the OS for now.
Allow overridding JS Alert behavior

Popping a BAlert on screen isn't very useful when running the testsuite.
Have the page send a message to the listener instead. Default listener
in BWebWindow still shows the alerts, but this can be replaced with
something else if needed.
Remove annoying printf.
Force SHARED_CORE for Haiku.

* DumpRenderTree can't work without it, and we would rather use what we
test.
Add information about the AHEM font to README.
DRT: Add Alert/Confirm and pixel dump support,

Also change default font size to help reusing test results from other
platforms.