(Bug 52422) Tweak PreHandler to handle WS-only lines between pre-blocks

* Lines with leading-whitespace (and other sol-transparent content)
  should not split indent-pre blocks:

  " a\n \n b" should generate a single pre-block.

* Added new parser tests covering this test and other edge cases.
  Some previously failing tests now pass because of the tweak.
  The new edge cases don't pass yet.

* Some selser change tests regenerated because of change
  in test titles.

* Verified fix on fr:YAML

Change-Id: Id3627c758a1ab8f11373f230ed92318c2e180a88
Merge "(Bug 52422) Tweak PreHandler to handle WS-only lines between pre-blocks"