(Bug 48231): Dont add new nodes in fosterable posns in DOM passes

* During encapsulateTemplateOutput and findBuilderCorrectedTags
  DOM processing passes, dont add new nodes inside a table in posns.
  where it will get fostered out by the client (ex: direct children
  of table, tbody, or tr).

* Tested encapsulateTemplateOutput behavior on following snippets:

1. <table>{{echo|  <tbody><tr><td>foo</td></tr></tbody>}}</table>
2. <table><tbody>{{echo|  <tr><td>foo</td></tr>}}</tbody></table>
3. <table><tbody><tr>{{echo|  <td>foo</td>}}</tr></tbody></table>

  In all these cases, where there used to be a span in foster-posn.
  before this patch, there no longer are any.

* Tested findBuilderCorrectedTags behavior on the following snippet:

{{col-begin}}\n{{col-3}}\nfoo

  The mw:Placeholder tag is no longer inserted in foster-position.

* No change in parser test results (TODO: add more)

Bug: 48231
Change-Id: I7e7fcef2f769c4d50c36384b264e212a833fe88b
Merge "(Bug 48231): Dont add new nodes in fosterable posns in DOM passes"