Fix "Undefined variable: LOG_FORMAT" notice

\Vimeo\ABLincoln\Experiments\ExperimentTrait#_log refers to $LOG_FORMAT,
when it should refer to $this->LOG_FORMAT.

Add the TestLogger logger to inspect what gets logged during the
ExperimentTest and NamespaceTest test cases.
Merge pull request #2 from phuedx/fixUndefinedVariable