Update the code such that when viewing a document in diffusion as remarkup, a table of contents (TOC) will automatically be generated, provided that the document has at least 2 headers!

Also fix t_block_interpreterests__/remarkup/interpreter-test.txt which was failing prior to this change (arc unit src/infrastructure/markup/remarkup/tests__/PhutilRemarkupEngineTestCase.php) - the block pattern match has been tightened up since this test case such that phutil_fake_test_block_interpreter is longer even recognised!

Fixes T15660

Diffusion:

• Create or update a markdown/remarkup document (otherwise an old cached version will be displayed)
• View it in diffusion
• Select the 'View as Remarkup' View Option (eg: https://we.phorge.it/source/phorge/browse/master/externals/octicons/README.md?as=remarkup)
• Confirm a TOC is generated

Diviner:

• Update a diviner document
• Run bin/diviner generate
• Confirm the diviner document TOC is unaffected

Differential:

• Create a diff with headers in the description. Confirm all works as before with no TOC.
• Create a diff comment with headers. Confirm all works as before with no TOC.