Async ref tests fail under heavily loaded machine

[bjwbell]

When opts.exit_after_load is set the compositor exits after the js load script completes execution. This does not guarantee that reflows have completed.

Affected ref tests (src/test/ref):

• append_style_a/b.html
• img_dynamic_remove.html
• upper_id_attr.html

To observe failures execute make check-ref-cpu on a heavily loaded machine, e.g. while compiling another instance of servo.

Firefox reftests, README Asynchronous Tests, have an attribute to tell the test harness when rendering is done. Servo could use the same solution.

Related issue #2871

[bjwbell]

No longer occurs on latest master. Possibly fixed by #3806, #3714, & #3759.