Two known failing tests for an Emacs auto-fill bug

Two known failing tests for an Emacs auto-fill bug

auto-fill-mode doesn't properly account for the fact that the first-line
prefix may not be the same as the fill prefix. Thus, it ends up breaking
before the first word of a list item, blockquote, etc. even though the
resulting subsequent line is no shorter.

See the following Emacs bug report for additional details:
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=20774

You can reproduce it as follows:

$ emacs -Q -l markdown-mode.el
M-x markdown-mode RET
M-x auto-fill-mode RET
M-x set-fill-column RET 5 RET

Type the following into the buffer and hit RET:
* Item

Actual result:
*
  Item
  [cursor]

Expected result:
* Item
  [cursor]

This commit adds two tests, both known to fail, as a way to track this
bug from markdown-mode.

Thanks to Sam Freilich <sfreilich@google.com> for these tests and the
bug report.