WTS: Verify source separator is valid before emitting it.

* The wikitext below has fosterable content in the tr-row
  and it gets fostered out of the table which means the DSR gap
  between the "[[x]]" and the body node will span table wikitext.
  This means source separator output will contain non-separator
  text and add spurious content to RT-ed output.
    {|
    |-[[x]]
    |}

* This patch verifies that the source separator is actually
  separtor text (whitespace and/or comments) and discards it if
  it is not. This prevents RT corruption in the presence of
  incorrect DSR.

* No change in parser test results but some improvemement in
  output of failed tests.

* Improves RT output of en:Sanjay Kapoor

Change-Id: I3ebadccd6643e4bbcde179029253bf8d45a551f7