Allow —no-dev install via a PHPUnit shim

Partially reinstates the 3.x style PhpUnitWrapper which was removed in d1af214ef5efb817c14b85b639525104d94ef2d9.
While we no longer need the full wrapper, the part which creates a fake class is still useful.

The preferred alternative would be to remove any references to SapphireTest from non-dev files,
which mostly applies to SapphireTest::is_running_test(). This should be solved in a larger refactor
of SapphireTest into optional traits and more fine grained functionality.

composer.json

@@ -74,7 +74,10 @@
       "SilverStripe\\View\\": "src/View/",
       "SilverStripe\\View\\Tests\\": "tests/php/View/"
     },
-    "files": ["src/Core/Constants.php"],
+    "files": [
+      "src/Core/Constants.php",
+      "src/Dev/PhpUnitShim.php"
+    ],
     "classmap": ["tests/behat/features/bootstrap"]
   },
   "include-path": [

src/Dev/PhpUnitShim.php

@@ -0,0 +1,14 @@
+<?php
+// Ensure this class can be autoloaded when installed without dev dependencies.
+// It's included by default through composer's autoloading.
+// class_exists() triggers PSR-4 autoloaders, which should discover if PHPUnit is installed.
+// TODO Factor out SapphireTest references from non-dev core code (avoid autoloading in the first place)
+namespace {
+
+    if (!class_exists('PHPUnit_Framework_TestCase')) {
+        class PHPUnit_Framework_TestCase
+        {
+        }
+    }
+
+}