Add some features to error handling.
replaced assertions using file_exists with phpunit's inbuilt
assertFileExists and assertFileNotExists
Merge pull request #6556 from CurosMJ/filesystem-tests-patch

FilesystemTest patch. Now using assertFileExists and assertFileNotExists
Add $key $value support to contains and where methods
Merge pull request #6553 from JosephSilber/where-contains-key-value

[5.0] Add where method to collection