When a developer tags a plugin we will prepare the plugin for download on the Marketplace. Before packaging / building the ZIP we should remove all tests from the "tests" and "Test" folder. Otherwise the included PHP files could be executed. This should not be a security problem as it would only trigger a fatal error as it would not find any other classes and there is usually nothing to execute but could potentially be one if someone is executing plain PHP code in one of his test files. This packaging behavior would be also consistent with the one in Piwik itself.
was implemented recently