Use the configuration driven unit test engine

Summary: Ref T5568. Use the very-new, very-beta `ArcanistConfigurationDrivenUnitTestEngine` unit test engine.

Test Plan: `arc unit`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T5568

Differential Revision: https://secure.phabricator.com/D13853

.arcconfig

@@ -1,5 +1,4 @@
 {
   "phabricator.uri": "https://secure.phabricator.com/",
-  "unit.engine": "PhutilUnitTestEngine",
   "load": ["src/"]
 }

.arcunit

@@ -0,0 +1,8 @@
+{
+  "engines": {
+    "phutil": {
+      "type": "phutil",
+      "include": "(\\.php$)"
+    }
+  }
+}