Tests for expressions in the attributes of custom tags

Test situations involving expressions in the attributes of custom tags. This has been implicitly tested in many places, but explicit testing is needed of all custom attributes with both types of expressions (handlebars and template string) wherever applicable.

For clarity, here is an example of the situation described:

<m-var component-name="foo-component" />
<m-var component-path="components/${ $.componentName }/fragment.html" />
<m-fragment src="{{ $.componentPath }}" />