Sortable: Write Unit Tests Covering #8572, #8573, #8574

#8572, #8573, #8574 was complex change that broke floating sortables. The floating issue is now fixed and covered, but there's nothing preventing these from regressing.

Here's the fiddles to use as the basis for tests:

#8572:

http://jsfiddle.net/Zvvpt/15/

1. drag the first item and move slowly down, when getting to the bottom of the sortable, the placeholder moves to the top of the sortable

2. drag the second item and move slowly down, the placeholder doesn't move until the item is dragged out of the sortable, where the placeholder jumps quickly to the bottom of the sortable and then goes to the middle again

#8573:

http://jsfiddle.net/6bsGx/3/

1. Click near the bottom of an item and move downwards, notice that the item can't be dragged out of the inner sortable

2. Click near the top of an item and move upward, notice that the item can't be dragged out of the inner sortable

#8574

http://jsfiddle.net/k4nn3/3/

1. Drag an item down, notice that it's hard to put the item between the two inner sortables.

2. If you clicked the top of an item and drag it down , it can be put between the inner sortables, but you can never make it there when moving upwards.