Tests for outline minor mode and visibility cycling
Fix bug in header visibility cycling

The level of headings was being calculated incorrectly because the
match group numbers were not updated to match the new regular
expression introduced in commit
0a2f5e0618bacaed2519be89aed3a4b0ce10a97f.
Update tests to use range-testing functions
Add test for setext-style header visibility cycling
Add variable `prev` to let-binding in markdown-next-list-item
Replace string-to-int with string-to-number
Thanks