feat(material/testing): Add icon name filtering to MatButtonHarness

[wildcardalice]

In its current implementation, icon buttons and FABs are more difficult to filter for compared to text-based buttons, which can use their { text} filter property, usually relying on a custom selector (eg: class or id) or ancestor. This change aims to simplify this by allowing a user to filter via a contained icon. The filtering logic is quite simple and reuses the existing logic inside MatIconHarness for simplicity and reliability.

Example usage:

const iconButton = await getHarness(MatButtonHarness.with({ iconName: "favorite" }));