Test for the targeted attribute, not the __default alias

[chriszarate]

Description

This commit introduced a change to parsed block bindings. It no longer provides the binding under the __default alias and instead only provides the actual targeted attribute (e.g., content for a core/paragraph block).

To be honest, this is a better result for Block Data API consumers, because they are not exposed to an alias that they would need to resolve themselves. Although, to be clear, this is all very theoretical. It is highly unlikely that any API consumers would inspect the bindings at all, which live in the metadata attribute. Almost certainly, they are only interested in the actual attribute value (e.g., content), which is unchanged.

[chriszarate]

Just a note that this test will fail for WordPress versions >= 6.6 that do not include this commit, which is currently all releases >=6.6. But those releases are not in the test matrix.

[alecgeatches]

Thank you @chriszarate! Merging.