Remiving fixture handling code from TestCase and removing compat methods

Now that we use phpunit directly both in the core and userland, we can
completely remove the fixture handling code out of the TestCase class.
Also removed some methods that were there for compatibility with
SimpleTest
Web test runner is no more, removing session related code
Removing obsolete test
Merge pull request #3077 from cakephp/3.0-testcase-cleanup

3.0 testcase cleanup