CON-644 | Slow tests list

This pull request adds displaying number of tests marked as slow in each test class.

Additionally it introduce new command ```./php-slow-list``` which lists all test cases marked as slow. More info in Readme https://github.com/Wikia/app/blob/94f3c99e11615ccb6bbc6676eeadb0233a4765a2/tests/README.md#running-only-fast-test-suite