Properly isolate easyblocks unit tests

[geimer]

With EASYBUILD_MODULE_NAMING_SCHEME=HierarchicalMNS set in the environment, about half of the easyblock unit tests fail as they are expecting the "classical" EasyBuildMNS. Similar to the framework unit tests, the easyblock unit tests should unset all EASYBUILD_* environment variables and honor TEST_EASYBUILD_* variables instead (cfr. easybuilders/easybuild-framework#1167)