simple-markdown tests: test loose list edge cases

Summary:
Resolves lack of tests from
c32d3d42ae7e80dcfe0d66aa8c8941124510d72e

This commit tests the edge cases when making lists
that have empty lines between some list items but
not all. These edge cases should have some
defined behaviour, but I'm not very concerned
with exactly how they function.

Test Plan:
`make test`

Auditors: emily