FIX: Throw more helpful error if tests are run badly.

If you run tests outside of a composer autoloader-based environment, or
your autoloader isn’t pulling in Constants.php, everything will break.

This provides a more helpful error message.

tests/bootstrap.php

@@ -3,6 +3,13 @@
 use SilverStripe\ORM\DB;
 use SilverStripe\Dev\SapphireTest;
 
+if(!defined('BASE_PATH')) {
+	echo "BASE_PATH hasn't been defined. This probably means that framework/Core/Constants.php hasn't been " .
+		"included by Composer's autoloader.\n" .
+		"Make sure the you are running your tests via vendor/bin/phpunit and your autoloader is up to date.\n";
+	exit(1);
+}
+
 
 /**
  * This bootstraps the SilverStripe system so that phpunit can be run directly on SilverStripe tests.