MDL-51236 phpunit: Prevent testsuite name conflicts

The testsuite name is fed straight into a class_exists() in
PHPUnit_Framework_TestSuite. In some cases, e.g. cachestore_static, the
class does exist, it's then fed into a ReflectionClass, and the test fails.

The testsuite needs to not conflict with any classes which could be
autoloaded in Moodle core.

lib/phpunit/classes/util.php

@@ -534,7 +534,7 @@ class phpunit_util extends testing_util {
 
         $template = '
         <testsuites>
-            <testsuite name="@component@">
+            <testsuite name="@component@_testsuite">
                 <directory suffix="_test.php">.</directory>
             </testsuite>
         </testsuites>';