Add test for setting properties with primitive values

Right now primitive values are allowed to be set as either an attribute or a property. Since properties and attributes are two separate APIs, custom elements could require that a value is passed in via one API or the other. I do think it's fairly common that there are properties not set from corresponding attributes.

There should be tests that set a property to a `boolean`|`number`|`string`|`null`|`undefined` and requires that the _property_ is set, not the attribute.