This is intended to provide an easy way for MediaWiki extensions to define pheaster tests that are run along with the ones for mediawiki core. The intended use is to set phester to scan the extensions directory, but avoid scanning all directoreis of all extensions, instead allowing extensions to specify if they define phester tests, and where.

Requirements: If phester encounters an extension.json file in a directory it is scanning for tests, it should examine that file, and look for a specific key (perhaps "phester-dir" or some such). If that key is not present, phester should not scan this directory or its subdirectories for tests. If that key is found, its value is taken to be a path relative to the location of the extension.json file; phester will continue the scan for tests there and ignore any other subdirectories of the dir containing the extension.json file.