Expose element in component integration tests

[Turbo87]

see emberjs/ember.js#14630

/cc @locks @rwjblue

[Turbo87]

Note that this is already exposed as the private _element property (see #172), but should be renamed to element to make it public API.

[les2]

@Turbo87 would it be necessary (or possible) to create a "deprecating alias" for the old property / maintain backwards compatibility?

my thoughts are that it's not required to maintain back compat because it is private.

but would any important downstream users be broken by renaming this property?

[rwjblue]

I'd prefer to preserve compat if possible. Seems like we can use Object.defineProperty getters or simply save the same element to this.element and this._element...

[rwjblue]

Closed by #220.