[wip] Add test info for softbreak

[chriseidhof]

This test actually passes. I would have expected it to have a different source range for the strong and text nodes, as per #296.

[honghaoz]

I believe this worth a test case as well.

  This is
    **strong**

Notice the leading spaces in the 1st line.

In addition,

1. This is
  **Strong**

1. This is
    **Strong**

1.   This is
    **Strong**

(Notice leading spaces in the 2nd line, seems it's related to the padding (3 spaces) of the parent list block)
These are some test cases I found useful to add.