A custom format is discovered only when both its table and index templates are present

All 7 scenarios hold

A directory contributes an output format when it holds both a table.<format>.peb and an index.<format>.peb template. A lone table or index template, other files, and subdirectories are all ignored.

expected outcome broken scenario

Scenario	Template Files	Discovered Formats?
Single format	table.html.peb index.html.peb	html
Multiple formats	table.html.peb index.html.peb table.xml.peb index.xml.peb table.json.peb index.json.peb	html xml json
Only table template	table.html.peb	[]
Only index template	index.html.peb	[]
Empty directory	[]	[]
Non-template files present	table.html.peb index.html.peb README.md config.yml	html
Subdirectories ignored	table.html.peb index.html.peb subdir/table.xml.peb subdir/index.xml.peb	html