Add failing test for default-valued properties

[ryecroft]

@attr properties with a default value (specified using a getter method) incorrectly have their setter method called with an empty string after setting the property in the connectedCallback

This incorrect behaviour does not happen if:

• the property is in the html definition eg <initialize-attr-test has-foo='' />
• the property does not have a getter method supplying a default value

[primer-css]

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[keithamus]

Thanks for filing this @ryecroft. I'll be looking into this soon hopefully!