feat(cdk/testing): make MatButtonHarness extend ContentContainerCompo…

[wagnermaciel]

…nentHarness

• Allows users to access mat-icons within mat-buttons

[crisbeto]

LGTM, but needs an update to the API goldens.

[josephperrott]

LGTM, just the one comment

[josephperrott]

Shouldn't one of these test cases remain as a mat-icon-button without a mat-icon element inside?

Suggested change

	<button id="home-icon" type="button" mat-icon-button>
	<mat-icon>home</mat-icon>
	</button>
	<button id="favorite-icon" type="button" mat-icon-button>
	<mat-icon>favorite</mat-icon>
	</button>
	<button id="home-icon" type="button" mat-icon-button>
	<mat-icon>home</mat-icon>
	</button>
	<button id="favorite-icon" type="button" mat-icon-button>favorite</button>

[wagnermaciel]

I wanted to have two mat-icon-buttons with mat-icons inside so that we could make sure that the correct mat-icon was being returned.

Also, the mat-icon with text in it was being used for any tests. @mmalerba What are your thoughts here?

[mmalerba]

I think the icon button with text in it was just a sloppy previously written test. I'm good with these changes

[mmalerba]

LGTM

[mmalerba]

Note for a followup: once #20646 goes in we might as well tighten it up so we're looking under the element where the content is projected

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