Discovers a custom format only when both its templates are present

All 7 scenarios hold

A directory gives the reporter an output format when it holds two templates: table.<format>.peb and index.<format>.peb. The reporter ignores a lone table template, a lone index template, any other file, and every subdirectory.

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