[Meta] 6.x Source plugin tests

The plugins are well-suited to a TDD approach, so we're beginning with tests first.

#2108753: Write PHPUnit tests for 6.x Role source plugin
#2108755: Write PHPUnit tests for 6.x User source plugin
#2108757: Write PHPUnit tests for 6.x File source plugin
#2108759: Write PHPUnit tests for 6.x Picture source plugin
#2108761: Write PHPUnit tests for 6.x Taxonomy term source plugin
#2108763: Write PHPUnit tests for 6.x Node source plugin
#2108765: Write PHPUnit tests for 6.x Comment source plugin
#2108767: Write PHPUnit tests for 6.x Menu source plugin