test/core_tests.js: Must catch events also when testing impress().init()
test/core_tests.js: Must catch events also when testing impress().init()

The first test block in core tests was just doing init with some
basic (synchronous) assertions. This had the effect that impress.js
would still be executing code in its own impress:stepenter event
handler when the second test() block is already calling next().

Catching impress:init and impress:stepenter after init() ensures
that we're really finished before moving to the next test() block.

henrikingo/impress.js,henrikingo/impress.js