simple-markdown: test some heading negative cases
simple-markdown: test some heading negative cases

Tests that

    heading
    --

or

    heading
    ==

don't make headings, and that

    #######heading

doesn't make an <h7>

See D12488 for more context

Test Plan:
`make test`

Auditors: emily

ariabuckles/perseus,sachgits/perseus,daukantas/perseus,learningequality/perseus,ariabuckles/perseus,learningequality/perseus,chirilo/perseus,ariabuckles/perseus,ariabuckles/perseus,abe4mvp/perseus,alexristich/perseus,daukantas/perseus,sachgits/perseus,iamchenxin/perseus,abe4mvp/perseus,iamchenxin/perseus,alexristich/perseus,chirilo/perseus