Add test for event handler content attribute handling for false positive event handler attributes

[lukewarlow]

See #520 for discussion on the loose spec definition for event handler attribute handling.

We should add a WPT that tests for attributes that follow the correct pattern of an event handler content attribute, but actually aren't. onreadystatechange is an IDL property related to XHR, this is currently protected by Trusted Types in Chromium, Firefox and as of WebKit/WebKit@4d3260a in WebKit.

As part of review feedback on the WebKit PR it was raised that WPT should probably test against this behaviour, such that we can fix it in future once we decide on the final spec behaviour (and fix implementations as appropriate).