Continue test harness execution if zone doesn't stabilize within a certain amount of time

[crisbeto]

Currently the test harnesses wait for the zone to stabilize before executing, however there are some cases where it might not happen. For example, in #23964 the test times out because a Material component uses requestAnimationFrame which the browser can decide to pause if it's out of view.

One thing to consider is that Jasmine tests time out after 5s by default so we should pick a number that is lower.