I don't exactly know what Core plans to do here, but
EntityUnitTestBase extends the old
\Drupal\simpletest\KernelTestBase), which is deprecated and scheduled to be removed by 8.2.0. Thus, in extension,
EntityUnitTestBase should probably be considered deprecated as well – normally, it would not be a problem if that class just were changed to extend the new
\Drupal\KernelTests\KernelTestBase) at some point, but in the case of tests this unfortunately would mean we'd need to move all our tests to different directories and namespaces as well, due to Drupal's separation between Simpletest and PHPUnit tests.
In Core, there isfor the migration, but it seems dormant for the moment. I've now asked there what contrib modules should do. Postponing this issue to wait for an answer there.
\Drupal\search_api\Tests\Processor\ProcessorTestBase is still uses
EntityUnitTestBase and it should use the new