the setUp() method has been declared public previously. while this is not
technically an error, there is no need in changing the visibility.
this commit addresses the issue by reverting test-cases setUp() methods
visibility from public back to protected.
additionally the BaseTestCase in Magento\Framework\TestFramework\Unit is
made abstract. this has been done because it would otherwise be identified
as test-case with no tests while it's only an abstract one.
further additionally some docblocks are added to address code sniffer
warnings.
fixes#2985
Lukasz Bajsarowicz