[Tests] Only use concrete TestCases

Summary:
Don't use abstract subclasses of ArcanistPhutilTestCase, only use
concrete ones.

This lets you put common functionality in an abstract BaseTestCase
(which itself is a subclass of ArcanistPhutilTestCase), then implement
concrete subclasses of the BaseTestCase.

Test Plan: Tested with a simple Base -> {Case1, Case2} setup.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2300

src/unit/engine/phutil/PhutilUnitTestEngine.php

@@ -118,6 +118,7 @@ final class PhutilUnitTestEngine extends ArcanistBaseUnitTestEngine {
         ->setLibrary($test['library'])
         ->setModule($test['module'])
         ->setAncestorClass('ArcanistPhutilTestCase')
+        ->setConcreteOnly(true)
         ->selectAndLoadSymbols();
       foreach ($symbols as $symbol) {
         $run_tests[$symbol['name']] = true;