chore(shadow-dom): add tests

[StrahilKazlachev]

Add simple tests for the shadow DOM <slot> emulation.
Left some commented out expectations. All related to unnecessary creation of the <slot>s fallback. The fix for nested <slot>s seems simple(#639). How to fix the same for the PassThroughSlot seems somewhat complicated, like:

<template class="slot2slot">
  <element-with-slot>
    <slot slot="named">
      <p>slot2slot fallback</p>
    </slot>
  </element-with-slot>
</template>

Did not cover usage of template controller as slotable like:

<element-with-slot>
  <p slot="named" if.bind="someCondition">Override</p>
</element-with-slot>

[EisenbergEffect]

@StrahilKazlachev Is this and #639 ready to merge as at least an incremental improvement?