test: prevent setting read-only properties on DOMRect

[straker]

Overriding the prototype for DOMRectReadOnly to throw errors on setting left, right, top, and bottom properties to match what happens in an extension context.

const test = $0.getBoundingClientRect();
test.right = 1;  // Uncaught TypeError: setting getter-only property "right"

This should fail tests any time we do this now. If I try to set the right property in get-background-color, multiple tests fail:

Closes: #3837