Event: Increase robustness of an inner native event in leverageNative

Event: Increase robustness of an inner native event in leverageNative

In Firefox, alert displayed just before blurring an element dispatches
the native blur event twice which tripped the jQuery logic if a jQuery blur
handler was not attached before the trigger call.

This was because the `leverageNative` logic part for triggering first checked if
setup was done before (which, for example, is done if a jQuery handler was
registered before for this element+event pair) and - if it was not - added
a dummy handler that just returned `true`. The `leverageNative` logic made that
`true` then saved into private data, replacing the previous `saved` array. Since
`true` passed the truthy check, the second native inner handler treated `true`
as an array, crashing on the `slice` call.

The same issue could happen if a handler returning `true` is attached before
triggering. A bare `length` check would not be enough as the user handler may
return an array-like as well. To remove this potential data shape clash, capture
the inner result in an object with a `value` property instead of saving it
directly.

Since it's impossible to call `alert()` in unit tests, simulate the issue by
replacing the `addEventListener` method on a test button with a version that
calls attached blur handlers twice.

Fixes gh-5459
Closes gh-5466
Ref gh-5236