Alter test runner to store `TaskMeta`

This patch changes `CeleryTestSuiteRunnerStoringResult` to store
`TaskMeta` results instead of `TaskState`.  In addition, it removes the
`on_failure` handling which could never have been invoked as implemented
anyway (as CELERY_EAGER_PROPAGATES_EXCEPTIONS was set True in the test
runner).

Addressing issue #191.