The current (at time of writing) manifest entry is (courtesy of bug 1222562, bug bug 982828):
skip-if = (toolkit == 'android' || os == 'mac') || e10s # OS X: bug 786938
This means we run it on... non-e10s linux debug (which afaict is the only non-e10s mochitest plain test env left). As far as I can tell we don't really test the handler service works in any meaningful way otherwise.
The other point here is that we seem to want to use a webcal: URI and some of that might stop working as a result of bug 1252831.
The test should be rewritten.