nowiki-wrap indent-pre-inducing sep space where possible.

* We used to unconditionally strip leading whitespace on newlines
  wherever it could have induced an indent-pre. However, when
  such whitespace showed up in text nodes with other content,
  it would get nowiki wrapped.

  This patch attempts to make the behavior more consistent by
  always nowiki-wrapping such whitespace (except in scenarios
  where it is not possible to preserve the whitespace.
  Ex: when serializing <ul> <li>foo</li></ul>)

* This exposed a couple other edge-case bugs in separator handling
  (use of strong-indent-pre-suppressing tags + setting of sol-state
  when whitespace only text nodes are processed).

* Introduce a new wikitext constant that specifies wikitext tags
  that are sensitive to space at start of line.

* A bunch of blacklisted test output changes.
  - Definition list html2wt and html2html changes are an improvement.

Change-Id: I387b2a155c3fcdde89d12da5d31cb135ebd04679