add test for ARIA PR #2210 ("Embedded Control" includes controls with value that are directly referenced by labelledby or describedby)

[pkra]

As requested by @cookiecrook.

This change to the "Embedded Control" step of the AccName algorithm w3c/aria#2210 already matches current implementations but would benefit from a WPT verification test here.

[cookiecrook]

@DanielRyanSmith can you transfer this to the interop-accessibility repo? Thanks.

[cookiecrook]

I pulled "2C" out of the title since there are only about 3 people on the planet that know what that means. "2C" is the legacy hash name of the "embedded control" step of the AccName algorithm: https://w3c.github.io/accname/#comp_embedded_control